Not because it is too violent. And not even because of its portrayal of mental illness, although that would be the easy criticism.
The movie is trash because it pretends that anyone becomes violent as a result of being victimized. No.
Trauma doesn’t cause people to become violent or abusive. Entitlement, resentment and believing in one’s own victim status cause violence–domestic violence, political violence, and the various sorts of school shootings and rampage massacres.
The movie would have been good if the Joker had never been the victim, but instead had always been abusive. And then found ways to believe he was the victim, by believing in his anger. That’s how the resentment necessary to carry out truly vicious acts of violence builds.
It sucks because that sequence where you see that the Joker’s girlfriend was never really there is one of the most powerful sequences I’ve ever seen. It’s too bad they couldn’t have been that real about violence, showing that the Joker was never really the victim.
But as it stands, the movie is trash.