Why the Mummy's are Useless?

Ok. Good luck waiting for the developers to talk to you personally.

Mummies are not useless. In the lower or midrange levels, they are gamechangers. In the high levels, other troops become more and more useful.

Mummies are certainly not completely useless on the offensive side. As some have said here, it spawns next to your king which can give you an advantage, especially if it’s a Surprise mummy! It’s a portable army of knights! 

Then we come to the mummies on defense. It’s one of the best defensive troops when boosted and TONS of players in the game use this troop on defense. Overall, I don’t think the mummy is useless on offense or on defense, but it certainly is worse than other troops on offense