What rankles is not the existence of mediocrity; by pure statistics, certain things will be in the middle range of quality. That is just the shape of the curve. What rankles is the holding up of the mediocre as some kind of model, and the social pressure that's involved in that. There was a guy in my department who was supposed to be great, etc... I guess that was the social fiction that we were all supposed to hold up. It was churlish of me to think he wasn't, since he somehow held a social status as being excellent, even though a view from outside that consensus might come to another position.
So the problem is not Billy Collins, but that idea of holding up Billy Collins as something special and exceptional. He's just not. When you point that out, people will say you hate him because he is too "popular.' Well, yes. that is why. If he weren't popular I would never have heard of him so I couldn't hate him. It is the lack of proportion between the merits and the reputation. That is what rankles.