I'm a data point on the side of YC not having an Ivy League (or "top tier") school bias. I went to community college. For audio recording and music--I've never taken a single computer class after high school AP classes. My co-founder went to a good school in Australia, but I didn't know what it was when filling out the application so I just said, "some school in Australia, probably a good one, and probably for computer science".

That said, the number of Stanford, CMU, UC, MIT, etc. students and graduates is quite high in YC, and I can't think of any other community college bums in the bunch.