This movie reveals a lot of uncomfortable truths about modern society in a way that makes it almost impossible to ignore. It shows how unfortunate circumstances and coincidences can create a monster, and that we are all at fault.
Funny how you mention cels, I think many people who didn't watch the movie associate with celdom, but almost everyone who has seen it can relate or empathize with certain aspects of Arthur/Joker's character, showing that you don't need to be a member of the lncel community to appreciate this movie.
I noticed also that people all over the political spectrum praise the movie's message, from conservatives to socialists to libertarians. The only group that hates it is nu-left SJW types.