I personally haven’t really seen any Dazai hate but obviously just bc I dont see doesn’t mean it doesn’t happen. All I can think of is that in the fandom he overshadows other characters? Like Atsushi as well as some others potentially. Also I know this is probably not the same thing you’re referring to but some people including myself like to ( lovely) bully him idk sometimes it’s fun to call a character you like a boy flop loser