Let's not forget the fact, though, that Mayweather is 40 and hadn't fought in more than two years. McGregor is 29. Maybe a skilled boxer of the same age and size, with months to prepare, would make for a decent MMA match.
i don't even think age is much of a factor at all here.
asking connor to fight in a boxing match is like having a guitar player prepare for an audition on bass. it can be done, but don't expect the performance to be on par with jaco.
asking a boxer of any age / skill level to fight in MMA is like asking that same guitar player to be prepared for an audition on every instrument in a full symphony orchestra. it's just less realistic i think.