Puck, like most original members of Alpha Flight (before mutants were popular) was endowed with mystical powers http://en.wikipedia.org/wiki/Puck_(comics) and was never a member of X-Factor. Many members of all the X-Titles ever published have NOT been mutants. Recently Omega Sentinel, Deathlok, Hepzibah, Fantomex, or even Longshot, Juggernaut and the "bamfs" are NOT mutants. Or at least not on Earth-616.
Just saying... its up to interpretation.