A nice guy is a beta male who acts agreeable and pleasant towards either sex, often shy, socially awkward and bad at flirting. Blackpillers and redpillers accuse nice guys of being bluepilled about women's mating preferences, and predict their best hope is to be betabuxxed to a post-wall femoid, provided femoids are being encouraged to marry very late. Nice guy's niceness and tendency to smile is commonly explained by low dominance status, neuroatypicality, physical weakness, religious or gynocentric domestication, or a genetic disposition for shyness. Many nice guys may genetically be slow life history strategists, programmed to wait to be married which never occurs in modern society.
Fake nice guys
Both womanospherians and manospherians accuse many men of being "fake nice", but in different ways.
Womanospherians seem to have a problem with virtually all genuinely nice guys, but will label them as Nice Guys™ nonetheless (see /r/niceguys). They deem all actual nice guys as fake nice guys, distracting from the fact that women reward anti-social behaviour. They see it as hypocritical when nice guys get angry at the fact that most women in the Western world do not find nice guys sexually attractive. Because anger is often not nice when directed outwardly.
Womanospherians also have a problem with actual fake nice guys, but so does everyone else.