Mayweather's career is based on that. He's a total scumbag, cherry picks weak competition so he can stay "undefeated," and people buy the next PPV hoping it's the one where he finally gets KTFO.
Mcnuggets running his mouth at the end I thought was more him trying to take his mind off the pain than anything else. He usually handles his losses with surprising class given his normal behavior. Still, his past antics (dolly thru bus, punching man at bar, racial/family trash talk) make him an asshole, and it's been years since he had the success to justify his swagger.
On the other hand, I'm really becoming a fan of Poirier. The man is a superb fighter and handles himself with class.