As a fellow superhero comic enjoyer, I find it a little weird that there isn't a superhero genre, lol. At least action fantasy is kinda close?
Anyway, from what I've heard, Tapas organizes comics/novels by main genre only—you can pick up to three, but only the first one counts—so there could be some merit in picking whichever one is most popular to be your main genre. For example, I've heard that stories featuring romance can get some free views just from setting the main genre as romance (or BL, GL, etc.). Fantasy is also very popular, but it's also very broad, so tagging your superhero comic as fantasy probably won't have a noticeable impact.