system
Given the following post, respond as if you're commenting on Reddit:
human
[Batman] Has the Joker ever been given a legitimate, professional diagnosis?

Various claims have been made about the Joker's mental issue. The comics often say he's "psychotic," but Joker doesn't have the hallucinations that accompany that illness. Some adaptations refer to him as having one or more Cluster B personality disorders, usually Narcissistic Personality Disorder or Anti-Social Personality Disorder ("psychopathic").^(❉) And yet these diagnoses don't hold up either, as Cluster B disorders are correlated with low intelligence, which doesn't apply to someone considered a "criminal genius."^(✲)

So what is really going on in his head? Is the Joker actually mentally unwell, or is he just some asshole who fakes having an illness to avoid being put in an actual prison? Is he actually a mentally sound person whose actions have a "rational" (from his perspective, not ours) motive?

❉ Oddly enough, nobody ever accuses him of Histrionic Personality Disorder, despite having an unhealthy obsession with getting others' attentions.

✲ It's worth noting that claims of real-life "genius psychopaths" tend to start with the disordered person themselves, who are trying to justify their own mental issues to themselves despite having no actual proof of their self-diagnosed "genius."
gpt
Short answer: No.

Somewhat longer answer: The people who have tried diagnosing Joker don't know what they're doing with him, and he just keeps tricking them into giving him whatever they think he has. 

Long answer: The Joker is such a bastard that it is unironically impossible to determine what is wrong with him. Every single attempt at it has been shown to be either factually incorrect within the same story or just a part of a much larger system. The common methods of diagnoses do not apply to the Joker as he is the most ridiculously complex case study for illnesses and behavior cancelling each other out *and* enabling each other at the same time.

In that sense, Joker's specific set of neuroses is its own thing. The best diagnosis would likely be calling it Joker/Napier Syndrome.  

It is categorically, demonstrably a waste of time and effort to try to pin Joker down with something that already exists. It is also unsafe and *will* make you a target.