He's definitely not cast as an evil person, but similar to prominent rich people who credit their success only to their own talents and not privilege or social circumstance. They beleive they hold the solutions to societies problems as saviors. I've never seen Thomas portrayed like that and I liked it for the most part.
That cheering is part of the issue, I thought the movie spared us from killings we felt were truly horrific. Everyone Joker killed in some form "deserved" payback, if not necessarily in death.
The Wall Street guys were jerks not just to him but to the lady, and flat out assaulted him.
His mother lied to him and contributed to his abuse.
His coworker ratted him out to his boss, almost set him up.
Murray brought him on to ridicule him after making fun of his videos.
In real life few would feel that murdering these people is justice, but in the context of a movie, all except perhaps the mom have an element of satisfaction. If he had killed the girl he fantasized about, perhaps someone in the audience, we might have felt that he was finally evil and lacking empathy, not just mad at the world.
I think the film would have worked better if more than just his romantic episode was shown to have competing narratives.