While Joker may be based on a comic book character, the movie is designed to feel as realistic as any character based drama. In at least one respect, it seems the film has done a good job and accurately representing reality, as a man who suffers from a similar disorder to that of the character of Arthur Fleck, says Joaquin Phoenix does a realistic job of portraying a man suffering from something that causes him to burst out in uncontrollable laughter.
In Joker we see the character of Arthur Fleck begin to laugh out loud on a bus shortly after he's been admonished by a mother for interacting with her son. While laughing, he hands the woman a card which states that he can't help himself. He suffers from a neurological disorder which results in the laughter.
Scott Lotan suffers from the pseudobulbar affect, also called PBA, as a symptom of multiple sclerosis. He tells LADBible that his episodes of uncontrollable laughter can last for 10 minutes at a time. He says the condition has forced him to be separated from family and friends during events like the wake for his own mother and fiancee who died in a car accident.
Lotan says that all the details of an episode are accurate in the film. Joaquin Phoenix even adds an element of choking to his laughter, which Lotan says really happens as he attempts to catch his breath while being unable to stop the laughing.
It looks like Joker did its homework when including this element of Arthur Fleck's character in the film. Representation matters and so it's nice to see both that Joker dealt with a real disorder and also dealt with it in the right way. While the concept of a man who randomly breaks into uncontrollable laughter certainly makes sense for a movie called Joker, this wasn't just thrown in as a plot point because it happened to work within the story.
In the movie, there's a question of how much of the story we're seeing is actually real. There's even a moment in the movie where another character asks Arthur if the laughing episodes are real or if he's just faking it. It's a question the film pointedly avoids answering. Although, we do learn that, again, assuming that anything in the story is on the level, Arthur was the victim of abuse, and so it does appear he may be dealing with PBA as a result.
Whether or not the laughing is "real" within the context of the story or not, it seems the condition as we see it is as close to the real thing as possible. Certainly this is a condition that we don't see represented in media often, and so it's nice to know that the time that it does happen, it's given the proper treatment.