...mediocrity is where most performers exist: midway between extremes, neither brilliant nor excessively dull. One does one's best with teh gift one is given. The point is that to say yes to that gift as it really is, is to say yes to its limits - and yes to the honest work that hones and polishes the gift. This is not the anxious, angry work that is really a denial of limits: 'If I just do a little more, then I'll be able to..." A Salieri can never become a Mozart. To say yes clears the air so that one can see and rejoice in things as they really are.
From Performer as Priest and Prophet, chapter 4 (pg 75), by Judith Rock and Norman Mealy, published Harper & Row 1988
I don't think the authors mean by 'mediocrity' a state of poor artistry, but of the 'mean' between brilliant and excessively dull.