I don't agree ...
Tristan doesn't mean "sad." It's the same name as
Tristram and
Drustan. And it's not gender neutral - some people seem to think it sounds like
Kristin and have given it to girls, and you may be right that that's why people don't use it that much - but that doesn't make it a genderless name.
Trista is a made-up name. I word-associate
Tristan with "trust" rather than "tryst."
Mallory isn't even really a first name, and I don't think it's comparable, since its familiarity as a first name originates with a 1980s TV show and not from well-known literature.
- mirfak