Yup, or as members of the Serpent Society or Mutant Force, in Captain America's books.
Asp (bio-electricity like Stinger, Bolt, Surge or Dragoness) always was, but then Black Racer (super speed), Puff Adder (weak size change and acid breath) and Rock Python (rock hard skin) were all either said to be, or alluded to be.
Plus those later additions to Mutant Force, when they became the Resistants; Rust, Quill and Mist Mistress.
It was never really clear to me if Fixer's brain was a mutant thing, or one of those 'inexplicably smarter than a dozen engineers strapped together, but totally human?' things. Since he regularly teamed with Mentallo, a known mutant, I always kinda felt he was a mutant smart-guy (like Smart Alec) and not just 'randomly superhumanly smart for no clear reason.'