As a rule of thumb, when a creature needs 20+ levels of off-page experience gain to be anywhere close to MitD-worthy, I call that "potted plant syndrome". As I demonstrated here, anything can be made MitD-worthy with enough levels/templates/additions. But at that point, how much is left of the original base creature? And thus, how can you be sure that base, rather than anything else, is the answer?
For levels in particular, we know that MitD was in a jungle doing nothing much for much of his life, then in a circus doing nothing much for some 20-odd years, then working as the monster in the shadows who is not allowed to do much until now. There is simply no justification for MitD having gained even a single level in his entire known history. Yes, we cannot outright discard he could have gained 20 levels off-page, but it is a hypothesis with absolutely no supporting evidence, I'm afraid.
Grey Wolf