Wouldn't he be completely outclassed in MMA?
Mayweather may be a hell of a boxer, but MMA guys are well-versed in many disciplines and unlike boxers, use fist, feet, knees and elbows. They also do moves (takedowns, submissions) that are not allowed in boxing. More than that, MMA fighters are used to having all these weapons at their disposal, unlike boxers who have to make do with punches.