Such a vulnerable civilization would likely find it very difficult to grow and develop in such a hostile environment, especially with a very small k-selected workforce. Elves have their speed and forests, dwarves have their brawn and mines, high beavers have their rivers and dams. What do mummies have to work with? Sand...?
However, their poor location would also give them access to a valuable asset that may make up for this weakness: r-selected races that are naturally forced into badlands by other k-selected races (such as orcs, goblins, or gnolls) would be a perfect source of a servant/slave caste to build their cities for them. Of course, they'd need to manage the dynamic very carefully since a slave revolt would probably be the end for the mummies, but the bedrock for a strong civ is there.