Deserts are harsh environments to survive. While in my opinion minecraft could use way more animals (especially predators) it would make sence if there where less spawns and creatures in the desert. I can see the mummy beeing the special mob of pyramids but it wouldn't make sence to have them spawn like an army.
the mummy should spawn near the desert temple well because it should be a challenge and other mobs like lions and cheetahs and other desert mobs are in a mod pack.
Edit: the animals that i mentioned is from Lots O' Mobs