In Joker (2019), we know that Arthur suffered from mental trauma in his childhood which affected his personality throughout his life. One of the consequences that we see is his unusual laughter; although mistreatment from society is also one of the reasons responsible of Arthur's present state: a person of such a jolly nature who likes performing for the ill children, who goes on to kill his mother along with half a dozen deaths of other innocent civilians.
If we dig a little bit deeper about why society was behaving badly to him, one of the reason that I see is they were not aware of the fact that there can be a psychological disorder causing a person to laugh continuously. Now, he did carry an explanatory card with him, but most of the time people were not aware of such a neurological condition and would start thinking he was making fun of them. Remember the reason for killing three rich guys in the subway was that as well. Therefore, it helps to ask exactly what kind of mental disorder Arthur was suffering from.
Movies often exaggerate the mental disorder and I think there is a risk involved with labeling someone while keeping in mind a movie reference. I don't know whether this is really a right question to ask here or not as it goes more into the domain of psychology and neuroscience. That might be the precise reason why director didn't name the disorder exactly. It's true that it may not even exist.