Or is adult just restricted for Sexual games?
https://en.wikipedia.org/wiki/Video_game_content_rating_system
The rating system on itch is not a rating system. But basically it is AO plus the M that might be considered "NSFW", and even T, if there are any that could have T, while also being considred "NSFW".
So, no, it is not restricted to sexual games, but because of the nsfw aspect, it appears so at first glance, as that one starts as low as some nude pixels. Also, there is the meaning in context of tagging and the tag "adult" is used as in the term "adult toy store".
So what defines a 'popular' game?
Clicks. The algortihm on itch adds some magic to it, that they keep secret. But it also involves a boost for new games and very likely mechanisms to not count artificial clicks.
I blame the youtubers. They make playthroughs of horror games and send their viewers here. I do not say this is a bad thing, but it surely boosts popularity of the genre.