Okay, so I love art. The first thing I do when I get a new RPG book is check out all the art. Then I'll go back and decipher all the lines of little squiggly black things which I'm told are called "words". Anyway, I've found that in most RPGs the people of a particular culture don't necessarily have a recognizable clothing fashion.
In the ancient/medieval world (and the bits of today's world which still haven't moved ahead) each culture had very distinctive and limited clothing styles. Take a look at costuming books and you'll find that most cultures only had one or two basic styles, and in some cases only in a very limited set of colors. I really think that if one is putting together an RPG campaign world then the clothing styles of the major cultures (with visual references) should be part of the design schedule.