It depends on the comic, and what part of the work is being asked about.
Some superheroes, like Batman or Spider-Man, have mundane or fairly flimsy technobabble-based orgins ("I will train and dress up as a Bat", Peter Parker is bitten by a radioactive Stan Lee) and others like Green Lantern or the Fantastic Four, are clearly science-fiction (cosmic police, a flight into space to gain their powers). Also, pretty much all superheroes have been in some sort of cosmic event storyline at some point (The Infinity Gem saga, Crisis on Infinite Earths).
At the same time, there['s really not a clear dividing line. If a question asks, say, about Galactus's origin, sure, that's on-topic. Asking about how nobody recognizes Clark Kent to be Superman with a squint would be clearly off-topic. Superman's origin is clearly SF, but his work at the Daily Planet kinda isn't.
In summary, I suggest we err on the side of being inclusive, and in general allow questions that ask about the science-fiction aspects of comic books.
(If the comic book proposal becomes a site, we can migrate questions there at that time.)