Because the fact that it promotes incel behavior (his flat-out creepy behavior toward the lead woman), shits on people with mental illness (yet another Hollywood movie that says everyone who is mentally-ill is predisposed to violence), and is flat-out unrealistic in terms of police procedures?
What I mean by the last one is that the Joker should've been arrested long before he killed the talk-show host: killing the three Wall Street guys on the train (he was dressed in the same suit, there was a witness, etc.), stalking the Waynes (and assaulting Alfred), assaulting a person at Arkham Asylum, killing his mother (he was the only person in the room), and killing his former coworker with a witness.
There should've been a whole squad of officers ready to arrest Joker, not just the same two that questioned him. So unrealistic.
Yes, that's another one I hated. I thought that the whole plot was cliche, and I disliked most of the characters (except for Michelle Rodriguez's pilot, and only because she was hot).