Depends what kind of shape Mathis comes in. Although it rarely happened when Mathis bothered to watch his diet and showed up in shape around 220-230 he was a tough customer and definitely would have a good chance at beating Carnera, but the 240+ version of Mathis that usually showed up most likely gets stopped.
Nah mathis came in at 243 against Frazier and gave him a good fight. I think the scorecards were close until he got stopped in round 11. I'd take that version of mathis over Carnera any day
and Mathis weighed 256, hadn't fought in 32 months and he went the distance with 29 year old Ali. Mathis was a skilled boxer who won the Olympic trials, don't think he'd have a problem with Carnera.