That's a bad comparison to prove boxing is the best hands only style. Not only did he take on the 'boxing champ' but he did so in boxing rules. Meaning not just "only hands", but having to fight with all the other limitations of boxing. This means they've got to use specific gloves, that are different than the ones McGregor is used to, and the round system was boxing rounds rather than UFC rounds, which can mess with someone's stamina (which is already McGregor's biggest issue in the UFC).
To add to that, the two champs aren't on the same level. Mayweather has won 50 matches with no losses, while McGregor won 22 matches and 7. So you're taking someone who never loses against someone who does lose, and expect it to prove something that the guy who never loses won again? Add to that the guy who is 22 and 7 now was 21 and 3 at the time, lost this match and 3 of his next 4, so he just reached the dip in his athletic career when this match happened.
To add to that, one win/loss doesn't prove anything. That's the issue with style vs. style. Take pretty much any sport, and you'll see multiple instances per week of the better team losing to the worse team. That's why most sports don't have 1 game eliminations. Look at baseball world series or NBA finals from last year; both finals went 4 and 2. That means that twice the "worse team" won. If you wanted to prove anything specific here, you'd need to take multiple boxing champs, multiple ufc champs, and have them face off against each other multiple different times. Only then could you have anything statistically significant enough to make any sort of claim from.
To add to that, even if there were neutral rounds/gloves/other rules (keeping just hands), Mayweather and McGregor were equal in their respective sport, and managed to fight enough times, with enough other champs added in to determine which is better, it still wouldn't prove that boxing is the best hands only style. All it would prove is that boxing is better than MMA at hands-only fighting. Which is expected, since MMA doesn't try to be the best at any one thing-it shouldn't be the best hands style, foot style, throwing style or ground grappling style. So it's not a good benchmark for that, and does nothing to suggest that boxing is better than wing chun, karate, muay Thai, kalaripayattu, bando, or any number of other arts, when it comes to hands.