In terms of other Necropolis monsters, the mummy is a fairly solid walker. However, they ought to be abandoned by most armies by late-game, since half of a Necropolis' units are fliers, and a stack of mummies will just slow down the rest of the army at that point, where movement may be prove crucial to success. See the article on Mummy (H4) on Fandom's mightandmagic wiki.