In modern American usage, mediocre is a pejorative word. To Horace, aurea mediocritas was a good thing, but we translate that as "golden mean" because mediocrity is, by definition, bad. The sense of "middle" has been lost.
It goes deeper than that. Average is viewed as negative in American culture. When people say mediocre they are thinking "eww average" in the back of their mind.
