All fetishes are popular, but its the uncommon ones that'll make you a bunch of commission money. Make an incest story, and you compete with hundreds of thousands of artists and authors. Make a belly-button worship fetish story, and you'll only compete with a few hundred artists and authors.
You've also got to weigh it against the demand side as well. There's far more demand for incest. So, as an author, if I write an incest novel, I'm competing with more authors. however, I'm also standing in front of more of an audience. If I write a necro novel, I'm standing beside considerably fewer authors, but also in front of considerably fewer potential payers.
From what I've seen, I'd (sadly) put loli over bestiality for popularity. I could honestly make a case for over incest even. people are disgusting for even considering underage shit like that arousing, though that might be a tiny bit hypocritical from me considering how far in i am with like submissive/dubcon stuff... anyhow my kinks don't matter here.
I never dove that deep into it. But I was ranking on a weighted scale - Popularity, ease of finding a place to post/find it, and commonality of the artwork/stories in that fetish. Loli was lower mostly because it is, thankfully, so difficult to find the fetish. most sites straight up don't allow the tag. Even SH here, where there is a Loli tag, doesn't generally allow it in smut novels. of course, it's more nuanced than that, but I'm speaking generally.