I'm not sure I've heard any claims that would point towards MtGox being a Ponzi scheme. It was a straight forward exchange, not an investment with returns of any sort, let alone those derived from later investors.

The allegations are generally much more straight forward, either some sort of incompetence in the form of transaction malleability, or some sort of malfeasance (pilfering the bank for operating costs, outright theft of the funds). Ponzi schemes these are not.