I think Floyd Mayweather would put up a better fight against Conner McGregor in MMA. Floyd Mayweather is a better defensive fighter, which is an important characteristic in any MMA fighter. Conner McGregor has the strength and skill to put up a decent fight, but I think he would be outmatched with Floyd Mayweather.
Mayweather would simply outlast him because of the endurance he has built up. It would be an interesting match to see them up against each other in MMA.