Ma Po Doufu without Pork
A number of Chinese restaurants are happy to prepare it without pork or beef. I've seen it with pork (most common) or beef (sometimes).

A vegetarian Chinese place that I occasionally visit uses a "vegetarian ham" along with some vegetables like peas to augment the custardy texture of the soft tofu. I like to add some ja tsai (zasai, depending on romanization preferences) pickles in there, though I'd guess that's not that common.

Pork is not absolutely essential to the identity of the dish, but it's certainly the most likely version to be served.