Genuinely, Mayweather with a year of MMA training would do better in an MMA fight against McGregor, than McGregor would do against Mayweather in a boxing match after a year of just boxing.
Now hear me out, I'm NOT saying he'd win! Just that he'd do better in comparison.
Duane Ludwig had a lot of success with simply sprawling and when that didn't work he simply tied them up in butterfly guard and either waited for an opportunity to kick away or held on until the ref stood it up.
McGregor isn't exactly Damian Maia on the ground and he'd likely try to keep it standing anyway. A year of training JUST sprawls, stalling on the ground and getting to know the distance with kicks is enough to allow him to hang with Conor and have some success of forcing it into a stand-up fight.
McGregor boxing on the other hand, would have to SIGNIFICANTLY improve his already best attribute to be able to beat the best practitioner of that skill set on the planet today.
Both, AT BEST, in those scenarios may be able to work for a decision in the other's respective sport. But McGregor would still be dominated and outclassed in boxing, yet Mayweather, if he could trade hands for even a portion of the fight would have some mild success (no more than a moral victory really).
Simply put...it's easier to avoid MMA in an MMA fight, than it is to avoid boxing in a boxing match.
All completely hypothetical of course since it'll never happen.