It's a shibboleth for ugly people; since they can't "win" on their looks, they opt not to play that game at all and need to loudly brag about their superiority over the vapid "fashion police".
It's a shibboleth for poor and dumb people; they don't have anything going for them (maybe they are not ugly but that doesn't help anymore after a certain age), so they need to play dress up to appear more than they are (and ever will be) and act like it too. Always funny to see.
