Being a genius is rough. You live in a different world, almost literally, when you consider how perception informs one's reality. Being surrounded by people who cannot keep up with you is isolating and frustrating. Being surrounded by people who are significantly less intelligent can be frustrating. There is a correlation between genius and existential depression, but that is situational, as far as there's plenty of research which shows a strong correlation between high IQ and existential depression.
Is "genius" just a mental disorder that happens to come with a package of extra intelligence? Are geniuses amazingly intelligent at the cost of sanity? Genius comes with trade offs, for most geniuses a sense of isolation is inevitable as how can anyone else understand what is going on; how can they even emphasize if they don't get the basic premises? I wouldn't say genius is a mental disorder but I would say it leads to isolation, and that can lead to mental disorders.
It should also be noted that geniuses who happen to have a mental illness will probably be noticed to a greater extent than average or dim people who have mental illnesses, because the combination of genius and illness will often lead to blatantly strange behaviors. Eccentric is a common term for those who fall into both categories.
Roughly 1 in 1000 people (depending on what arbitrary value you use) is a genius by mental capacity. However, that doesn't really mean anything. When people think of geniuses, they think of people like Einstein, Michelangelo, Da Vinci, etc. Most geniuses are not remotely like that. Most geniuses accomplish nothing of note and contribute very little to society. Meanwhile, most of the people who have won Nobel prizes would not score in the 99th percentile on IQ tests.
Genius as a means to describe mental capacity has meaning. However, genius has no meaning and is a fairly useless term beyond that. Being a genius isn't what most people think, and it is not that special.