If you ask a group of Chronically Normal People (CNP's -- a term invented by Fred Frese, a psychologist who has schizophrenia) to describe mentally ill people, you will inevitably get answers like "they smell,""they wear their clothes weirdly", and "they have wild hair." It would be nice to be able to dismiss these descriptions as purely prejudiced and totally unfounded. But the truth is, many mentally ill people are dishevelled and unwashed. The primary reason for this is, of course, the high rate of homelessness amongst the mentally ill. Homeless people have no easy way of keeping clean and they often have to wear all the clothes they own.
However, many mentally ill people look unkempt, even if they who are housed and have a job. Ordinary things can become monumentally difficult when you are mentally ill, and sometimes, the effort involved in taking a shower is just too much. Unpleasant thoughts and/or auditory hallucinations can be so distracting that the person can't pay attention when they put on their clothes, and so they may end up wearing a shirt inside out or leaving zippers unzipped and buttons unbuttoned.
Another reason why mentally ill people may be unkempt is that dressing carefully seems unimportant or even ludicrous in the mentally ill person's universe. If you are tormented by suicidal thoughts or living in a delusion in which you are being chased by the CIA, you do not worry about taking a shower. There are more important things to deal with, such as planning (or trying not to plan) a suicide attempt or evading your pursuers.
So what's the moral? Silly as it may sound to those who comdemn "political correctness," people who are "differently kempt" should be a tolerated and protected minority. When you run into someone who smells, it is rude to mention it or sniff repeatedly. That's just the way they are. You wouldn't sniff at someone who has a skin color you consider ugly. Likewise, making fun of or being rude to a mentally ill person who is dressed oddly is unacceptable. If you are a very, very close friend, you may want to tell the mentally ill person that their shoes are untied or that they have body odor, but be careful of doing this. The person may simply not have the energy or concentration to deal with such a thing. They may know already that they look funny and feel helpless to change their appearance.
I sometimes think there is a secret set of instructions on how to look normal that mentally ill people don't have. I was once called in to my boss's office to be told I didn't dress well enough when I clothes that were almost identical to my boss's. The problem was that I looked unkempt and sloppy in the very same clothes that made her look attractive and professional.