Mamoun's is popular only because it stays open late, not because it serves good food. Anyone who likes falafel or baba ganoush or hummus would look elsewhere (namely, One Stop Mart and Sahara Med in New Haven, or Olive Tree in Milford) for higher quality, better-tasting food.
All of the food here is mushy and watery and it's hard to distinguish between dishes, because they all blend together into an undifferentiated mass of chickpeas and eggplant.
The falafel here is laughable in taste (too dense, underspiced).
All that said, here's why Mamoun's warrants two starts instead of one: it's always open when you need it, and it's cheap (albeit cash-only, which always messes things up). Also, all the desserts are vegan (that means no butter, no eggs, no milk and no honey).
If you want good-tasting food, go elsewhere. If you are in a bind and it's late at night, Mamoun's will do the trick. Just don't expect much.