What exactly is Arthur Fleck's mental illness?

All of them. He has all of the mental illnesses.

- There's the uncontrollable laughter: the Pseudobulbar Affect. Sure. Let's say that they nailed that one down, and it could be explained from his brain injury.

- You could say he's a psychopath, which really isn't a mental illness per se. Only outside of the murders, he doesn't really show the symptoms of psychopathy.

- Schizophrenia? I'm not a psychiatrist or anything, but except for the hallucinations of Zazie Beetz, he doesn't really show any symptoms of that. The movie doesn't give us any, at least.

- PTSD. The past trauma is revealed, but we aren't shown any flashbacks or triggered episodes symptomatic of that.

None of these things except the first are actually stated in the movie. We're just left to believe he has whatever mental illness would cause him to act child-like and soft spoken, create his own imaginary girlfriend, and then start killing people, and then reveling in chaos. Which is bullshit and an insult to anyone with an actual mental illness.

You could say this is picky, but a movie about how badly society treats people with mental illness should at least tell us what mental illness the main character has, instead of basically shrugging and saying he has whatever mental illness makes him fucked up and cwaaaaazy.