The movie apparently wants to explore "Arthur's" issues with mental illness, and on it's own that's a terrible idea because trying to make a psychologically believable commentary on mental illness using The Joker, the farthest thing from a psychologically believable person, is disingenious at best and grotesquely offensive at worst.
But the movie is set in the 70s, and America in the 70s was faced with not only a spike in violent urban crime, but also Reagan's cuts to federal funding for mental hospitals which led to a spike in homelessness and people who needed treatment just getting funneled into the prison system. Given how this movie seems to be literally just Taxi Driver meets King of Comedy it seems like that's the idea behind their take on Joker. A failed comedian struggling with mental illness who fails to get the help he needs and bites back as a result.
I still feel like The Joker and every other element related to Batman comics is just something they are slapping on this movie to make it sell.