Actually, according to this site, it doesn't mean sad:
http://www.behindthename.com/php/search.php?nmd=n&terms=tristan&submit=Go
Well, the -an ending makes it seem wimpy to me. It's just a really weak syllable imo, because it's so common and seen on kids of such wimpy suburban white parents trying to be cute. (A little blunt, but, well, you asked :).) And part of it is how trist means sad. And short i isn't a very strong vowel in the first place. A name where the only vowels are short i and a schwa can't help but sound wimpy to me.