Reason that diagnosis from afar is illegitimate isn't just that you don't have enough information (but there's that). Just as important is that "real", official, mental health diagnoses generally don't make sense without the context that a given person isn't functioning in society, has violated some institutional norm, etc.

The role of mental health basically is to look at someone who's considered non-functional and classify how they're non-functional. If someone, mental health professional or otherwise, looks at someone in society and doesn't like how they're functioning but society is OK with this, that person can say society or some part of it is insane but collective insanity is a manifestly different phenomena than an individual diagnosis.

Edit: Behavior some consider bad, that some people get away with and that is actually prized by some other section of society (whether it be hitting on women or taking ultra-moralist stances or trolling generally or whatever) can't be official, institutionally defined insanity, even I don't might informally various actions "crazy" (which indeed occasionally offends people in the present context).

Edit2: Another I'd put is that the specific tools of the mental health professional aren't tools for understanding people in general and aren't tools for understanding bad behavior outside a context where it's debilitating bad behavior. In those contexts, sociologists, maybe, something specific to say but to a large extent, specialized knowledge by itself may given an advantage and someone tossing around psychiatric terms in this is kind of engaging in pseudo-science.