Oh, that sounds like it could be really useful for readers looking for a specific kind of story! Like if someone wanted fantasy-romance stories, it would be harder to find comics that fit both those genres because Tapastic only lets you put your comic in one genre. Or they could sort comics for more specific themes/genres, like lgbt, cyberpunk, pirates, etc.
Regarding what Puppisama said, maybe these groups wouldn't be social/self-promotional at all, and were just simple lists of comics with a certain theme, maybe with a moderator to make sure all of the comics actually fit in. It would be a little dull, but practical.