A lot of times your performance is not just based on how well you did but also how difficult your challenge is. Lets say you are studying a difficult course, I am studying an easy course relatively. You get a C grade and I get an A grade. This does not mean that I would have done better than you if I did the same course. Makes sense?
To relate this into boxing and MMA, a fighter can fight a lot of D level fighters and win in fashion/be undefeated. Another fighter could fight some good challenges, and might not look so good; may even take few loses. Who you fight makes a difference. This is why I don't buy into hype easily and pay attention to who you fought. Take a look at Amir khan and Kell Brook for example. Amir khan was fighting guys that were good to decent a lot of his career, since 2009 :
Kotelnik
Marco Antonio Barrera
Paulie Malignaggi (in his prime)
Marcos Maidana
Zab Judah
Lamont Peterson
Danny Garcia
Julio Diaz
Luis Collazo
Devon Alexander
Chris Algeri
Canelo
Meanwhile, Kell Brook was bragging about being undefeated before GGG and trying to state that unlike Khan he never lost and never been KO'ed. These are the best oppositions he has faced:
Senchenko
Shawn Porter
GGG
Errol Spence
REST OF THEM ARE POOR OPPOSITIONS, and I mean POOR.
So if Khan get's KOed more times before Brook faced GGG does that necessarily mean that Brook can take a better shot than Khan? That Brook is better than Khan? No it does not. After all, you can give me 100 very non-athletic and very weak people with no fighting background and I will be undefeated with 100 KO's. That would be a better record than all of the guys that I mentioned in this post but does not make me a better fighter than any of them.
So how does this relate with Mayweather. Mayweather's last 6 before McGregor:
Andre Berto
Manny Pacquiao
Maidana (fought him back to back twice)
Canelo
Robert Guerrero.
All of these are difficult KO. Therefore just becasue Mayweather hasn't KO'ed anyone recently does not mean that he doesn't have power. Andre Berto only Ko'ed when he had 1 hand vs Soto Karass and against Porter who headbutted him to a KO. Guerrero got Ko'ed as he start to fade away right before retirement.
All of a sudden McGregor who is dropping guys easily with punches is not able to punch as hard as Mayweather. It is because the people who he KO'ed in UFC is levels below Mayweather when it came to their boxing.