Mediocrity

The first point is that half the people are going to be below average, in any given endeavor.  There will be a lot of people clustered in the middle, on any bell curve, as well. If your definition of mediocre is below average, then that will be half. If your defintion is "middling," then you will get a substantial number. 

The second point is that mediocrity is defined contextually, by people already engaged in doing something. So a mediocre second-baseman will look mediocre compared to others in the same league, but still be better than anyone who couldn't play in the league in the first place. He had to have risen to that major league level even to achieve "mediocrity."  

Mediocre is pejorative adjective, that seems to mean "middling," but has come to mean "bad" instead. The connotation has usurped the denotation. This is because it is only used when something doesn't meet expectations for excellence. On a tenure committee I was on everyone was judged excellent by their departments, to the point that calling them "very good" was almost insulting, and caused heated debates. That is because rhetorically speaking nobody can be below average, even though we know honestly that there is a middle and a bottom to the curve, not just a top.  

When I hear the phrase "mediocre white man" I don't think of myself. Nobody does. Only other people are mediocre. If I object to the phrase, then I sound defensive.