I will say that in the first book he's about as much of a super HERO as super villian, in either case he still has powers beyond that of a normal human being. So, no matter how you slice it, he fits the definition. The reasons for the lack of costume are detailed pretty well in the books, so there's little reason to blab on about them now.
I'd also like to point out that I never call him a superhero in the books. NOT b/c he isn't one, but b/c there's a couple comic companies out there who like to sue for the use of that word. I won't name names, but since you have internet I'm sure you can find at least one news story out there about it. If you search "sued for use of word superhero" you'll find it pretty quickly. The More You Know (Cue rainbow effect)
I'm glad mutant hasn't been trademarked, at least I can still use that generic word! I'd come up with my own term, but you know as well as I do the criticism will be "blarg, you made up a term that already exists as something else, stop pretending to be novel, blarg." :)