Mediocrity is a worthwhile tactic if you're not optimizing for money or status, but, say, happiness. Ambition can cause a lot of pain, particularly in feelings that you have not lived up to your potential (I've seen plenty of that happening).

YC readers are understandably strongly biased against such values of take-it-easy life. I don't subscribe to such values either, but they can not be straightforwardly discounted as inferior just like that. I would argue that understanding your limitations is just as important as developing your abilities.