Man, there's a dedicated instance for yiff.
Compared to that, star trek is a million times more popular lol.
But, like other people have said, it's a huge thing. Star Trek is one of, if not the most successful TV universe. It crossed into movies, comics, books, toys, everything. The "brand" spans most of a human lifetime so far, and has done so with a surprisingly minor degree of fuckery that insults the fan base.
So you end up with generations of fans, across all kinds of demographics.
It's science fiction. And there's not many hives of scum and villainy places online that are more sci-fi friendly than lemmy; even mastodon isn't quite as sci-fi loving.
But you may be missing that star wars has a ton of related C/s as well. There's dedicated sci-fi C/s scattered around. Compare that to genres like fantasy or westerns, and you start to see the trend leaning that way.
But, you're right that it is a disproportionate thing. The only "brand" that seems to be as universal as trek on lemmy is LOTR, and that's a foundation of fantasy as a whole.