This all seems to rest on the assumption that more (drive-by, wooed by flashy logos) contributors => better software quality / functionality.

That's... quite a bold claim.

EDIT: And if you really want contributors, then maybe instead of building yet another logging or command parsing library with a cute mascot: contribute to and fix existing codebases, or discuss design and suggest changes to a standard library? There's plenty of existing communities of people that you can work with on a 'popular' library. If the target space is so crowded that what makes a difference is pure marketing, then maybe you don't need to make that space even more crowded in the first place...