First, your opinion on a movie is your opinion. I'm tolerant of films to a great degree. Ive enjoyed the campiest , goofiest films as much as any classic. That is to say, I don't proclaim to be some nuanced film critic.
My feeling on the bad reviews I'm reading is they are missing a key perspective. As
@baskinginthesun points out, the joker's creation is really the climax of the film. He isn't any sort of embodiment of the comic villain until the last fifteen, twenty minutes. I'm sure stemming from personal experience and thus empathy I was struck with little things throughout the film that touch give a bit more weight to the fact that mental health isn't just genetic. Early on perhaps we think Arthur's issues stem from the fact he's son of a clinically ill mother. But really there's the child abuse at heart...which even today is probably an aspect not understood well enough, certainly by laypeople (ie non child pyschiatrists). The being beaten up by kids and the wall st types is blatant. But I was moved by the multiple scenes in the clown agency. Ostracised in that group. The weirdo. I empathized with that. And I'm sure many who have felt strained with cliques and bullies at school could relate too. Now, not all of them go on to become joker. But you know what - they do go on to shoot up schools. They go on to join Isis.
Take the latter for example. I think it was Denmark police started a wildly 'radical' approach to counter terrorism. They tried to understand it as a mental health issue as opposed to the socio-political beliefs. Upon having leads on troubled immigrant youth, bullied and left out of their schools or neighborhoods, and partnered them with community leaders than integrated them into society. That prevented and even reversed their trajectories. The FBI is doing much the same with great success.
A lot of folks may not classify these as mental health. But I'm of the opinion they should. Emphasis on opinion. Because sure not all of these individuals become anarchy leading criminals or even school shooters. Many do things like develop eating disorders. Or cutting. Or doing drugs. Killing themselves. Hurting others.
And the joker represented in this film? He's an extreme. I mean, it's an iconic comic supervillain afterall. But the descent from child abuse victim to ill treated and isolated is taken to the madman level because that's what he is...a psycho character. But the subtle moments earlier are there. That public humiliation on the TV show of his bad standup versus in his own head his voice is clear (that first imagined scene on the show). It was reminiscent to me of Look Me In the Eye, the complex inner thoughts of an autistic boy growing up misunderstood and maligned. Just so many moments that came together. And the way a comic book is an exaggeration of reality, isn't that the perfect joker? All these issues compounded into one fecked up killer clown?
And that's before the socio political stuff...which Arthur even dismisses as not his personal cause. But it matters. It's at the heart of the environment that created him.