Evil moron theory is a subset of the "co-alpha male" theory which states that women in 'modern culture' sexually select for immoral idiots.
The theory states modern feminist society is inherently dysgenic, as women post-feminism are claimed to transition from selecting for high-status and provident men (or men that have genetic traits associated with these things) in favor of traits that are deleterious to the maintenance of civilized behaviors and social structures such as stupidity, impulsivity, laziness, low morals, promiscuity, and general 'free-rider' and anti-altruistic behavior.
In the process of this Fisherian runaway, it is claimed that highly moral and intelligent men are then selected against until this process causes economic and social collapse, and a rough sort of 'order' is then eventually enforced, where women again begin to seek out beta-providers and 'co-alpha' males. This is why advocates of this theory advocate seeking out women from 'post-game' third world countries as a solution for inceldom. Thus it is essentially claiming that women in a "healthy society" select for men with traits that are beneficial to society, while in "modern culture" it is advantageous for women to select for traits that are antisocial and anti-altruistic.
This conception of human mating appears to be inspired by the traditionalist cyclical view of history advocated with thinkers such as Ibn-Khaldun and Oswald Spengler.
Compared to other theories
It has similarities to some PUA type, redpill theories about women preferring the "caveman" or "gorilla" mindset (violent, thuggish men). It is mainly distinct from these theories in that redpillers claim women are primarily attractive to dominant, good looking and wealthy men, and that women are attracted to these men in part because of their 'high genetic quality,' whereas the evil moron theory claims that, in an era of feminism and promiscuity, women are most attracted to men who would be on the bottom of society or who likely wouldn't even exist at all in an environment of harsh natural selection. This theory asserts this is because women calibrate their preferences in men to those who have they observe generally have the highest reproductive success in a 'culture.'
In an era of feminism, modernity, a highly developed welfare state, etc., this theory proposes that women's genetic fitness can be maximized most effectively by breeding with fundamentally degenerate men, regardless of these men's socio-sexual rank or ability to lead and be respected by other men and so forth. PUA-types also claim that women are hypergamous in general, especially in terms of social status, education and financial success, all of which are moderately related to intelligence, while evil moron theory states they care much less about these things than their partner's (bad) personality.
The core idea that women's mate choices have switched from being eugenic to dysgenic due to modernity is similar to the assertions made by oofy doofy theory. However, the latter theory is less based on evolutionary biology and more on the claim that women's sexual preferences are more amenable to cultural programming than men's (an idea that evil moron theory also holds to an extent but it claims that this is because women are primarily concerned with maximizing fitness and due to their purported inherent tendency to submit to cultural norms more than men) and that women prefer these men largely because they make these women feel emotionally secure, etc., and certainly not for any ultimately rational biological reason.
Female attraction to evil in popular culture
In modern times, the popular notion of the bad boy "getting the girl" has a lot of origins in 1980s coming-of-age stories and teenage movies. Even the chad archetype is based off a caricature of a stereotypical jock from this time period. This idea is not necessarily modern though, as the concept of the "rakish" man (a morally dissolute man who is known for cavorting with women) who is popular with women has been around for centuries, ever since regency era England at least (the late 17th century). It is also noteworthy that the man whose name is now the byword for a master seducer, Giacomo Casanova, was a bisexual pedophile, a gang rapist, and a man who purchased a 12-year-old girl as a sexual slave during his visit to Russia. Casanova also subsequently seduced a woman who later "gifted" him her blatantly pre-pubescent daughter as a sexual plaything, an incident Casanova luridly describes in great detail in his famous memoirs. These kind of behaviors were clearly considered morally repugnant by established authorities in the era, despite Casanova also professing to be a devout Catholic.
Another contemporary of Casanova, the Marquis de Sade, was a sadistic pedophile who wrote infamous literature that primarily revolves around his fantasies regarding the prolonged sexual torture and murder of children. He was certainly an "evil" man by most moral standards, even exclusive of his own morally questionable fantasies and actions, as his core philosophy was based around desire being the ruler of action and any other forms of moral considerations being illegitimate, a stark rejection of all conventional morality. Notably, this concept was later formally encapsulated by the satanist Alistair Crowley's inversion of the Christian "golden rule", that is, "do as thou wilt is the whole of the law". De Sade was also the author of two novels, Justine and Juliette, consisting of the stories of two sisters, one virtuous (Justine) and one completely morally bankrupt (Juliette). In these novels, despite her virtue, Justine is throughout the course of her life subject to innumerable calamities and is prayed on viciously due to her sweet and innocent nature, while her wicked sister lives a life of hedonistic vice and success. As he was also a committed atheist, it is clear that the purpose of these novels is to destroy any non meta-physical argument for behaving in a moral manner as being personally beneficial to a person in a society that is mostly morally bankrupt, i.e. De Sade is overtly advocating for evil as being superior to good. In light of all this, it is probably not surprising to note that De Sade was a very sexually successful man, with him having a steady string of (willing and unwilling) lovers throughout this life span, including a 14-year-old girl he seduced shortly prior to his death in 1814 at the age of 74, despite him being old, virtually a dwarf (5' 2"), physically repulsive, morbidly obese (owing to his gluttony) and consigned to an insane asylum at that time.
So it's highly dubious to propose that only women in "modern culture", specifically, are attracted to such men or that such men have not always been highly attractive to women (perhaps in spite of their behavior). Though it is important to note that regency era England and the late 18th century Europe were noted for their relatively high sexual permissiveness, so it could be that women are merely calibrating their preferences to the kinds of men whose behavior is most in line with the fashion of the times.
Mainstream dating advice
Most of mainstream dating advice is based off the assumption that women prefer malignant or at least selfish men (as a sidenote, same with some female dating advice, which advises women to be bitches). However the mainstream conception of "women like jerks" is not presented as starkly as Fschmidt presents it. Modern dating advice doesn't call it "evil", and instead states there is moral virtue in becoming a selfish asshole, which is somewhat consistent with the objectivist roots of modern dating advice. Many of the promoters of this sort of dating advice are also likely (indirectly) influenced by the philosophy of Friedrich Neitzsche in the vague sense of them seeing it as proper and good to exert their "right" to exert their will to power by ruthlessly pursuing women, material goods, status as well them expressing a general anti-altruist moral bent that is also likely strongly influenced by social Darwinism. This worldview then tends to be infused into their dating advice, which generally takes the form of discouraging overly pro-social behavior in men (in the belief that this is a form of weakness), or at least maintaining that women are generally repulsed by the display of these traits, tacitly or overtly.
There is a large amount of modern psychological research that suggests that women are indeed generally attracted to men high in the "dark triad" traits of narcissism, Machiavellianism and psychopathy. Though the dark triad is a psychological construct designed to measure latent personality traits generally associated with socially undesirable behavior, it is not the same as the broad metaphysical concept of "evil". However, attempts to categorize and measure behaviors seen by many traditional religions as sinful or evil, however (such as the Christian seven deadly sins concept that is particularly prominent in Catholicism and the Eastern Orthodox Church), have found that the dark triad traits are moderately correlated with these sorts of behaviors.
The theory was formally developed by Fschmidt who started a subreddit for non-morons, called /r/nonmorons. Fschmidt's buddy Caamib also ran a forum for "nonmorons" (who he deems co-alphas), called the Co-alpha Reactionary Forum.