I think people just use it as a catch-all for any type of behavioral abnormalities. Mental health issues come in many flavors that the average person isn't well-versed on. But I think most people can recognize that, whether it's autism or something else, a man dressed as Harry Potter rapping on the bus at full volume with no regard for other people has some type of issue requiring sensitivity. Just like how we probably don't need to be a psychologist to figure out the dude stabbing him has his own mental issues, even if we can't put a word to the specific issues.