Depends whether or not it's comic or film Mystique, as well as if the modern Clayface is his comic or video game self.
Film Mystique is on a similar level of power to Arkham game series Clayface but is fast enough to blitz him and ought to win with mid-diff.
Post-Flashpoint Clayface is far stronger than Film Mystique and on a similar level of speed, so he should stomp. Against her comic self, he's still just as much stronger but Mystique is far faster, so it'd come down to who can wear the other out first. Clayface has enough range to keep her from getting close, but Mystique probably has more combat intelligence. Seeing as how Mystique isn't as well known for extensive fights, I'd probably say Clayface takes the comic matchup with high to extreme-diff.