I am, rather more than most around here, pretty well versed in psychology. At least given my interested amateur standing; do not take anything I say as a professional medical opinion, because it assuredly isn't.
That being said, yes, some of them are mentally ill, but no, most of them are not. I have observed what I believe to be signs of schizophrenia in a rare few of them, but there are a lot of other factors at hand. Some are ignorant; I would, in fact, say that most are ignorant. Well, we're all ignorant of something--of many things, even. The issue becomes deeper when the ignorance becomes willful.
"Now everyone was giving her that kind of look UFOlogists get when they suddenly say, 'Hey, if you shade your eyes you can see it is just a flock of geese after all.'"
"You can't erase icing."
"I can't believe it doesn't work! I found it on the internet, man!"