Err... possibly because both the wonder and destruction decks are pretty much crap, the former especially for mummies.
There's precisely one card that's worth a flip to a mummy in the deck of wonders -- experience. The rest just aren't useful. Unless potion cards work on mummies? Either way, potion cards just aren't terribly helpful -- if the wiki percentages are right, it's got a better than 1 in 5 chance of causing you to rot instead of doing anything useful if you're drawing from the second power level possibilities

. The only really good cards in a wonders deck is experience and helix, and mummies can't mutate. You draw three/stack five wonders decks. Getting a shuffle or focus card can bork your game right up right quick.
Admitably, the destruction deck is mostly harmless (you can still electrocute yourself to death), but Nemelex's real damaging power comes from summon decks; all mummies let you do is eat the destro decks for piety instead of just chunking them in a corner to keep from killing yourself.
You're after summoning (Nemmy summons can kill anything if you spit out the right critter/pack. Sometimes, they slaughter off an entire freaking level, i.e. red wasp swarm) and escape (triple draw or stack five), with a side eye out for legendary dungeon decks for trowel cards. Most everything else nemelex gives you is either something to farm for piety and more decks or throw in your stash to keep from using accidentally.
I'd probably suggest running a MuSu just to be able to support Nemelex's summons even more. You'll want to be saccing corpses more than potions, though...
Honestly, sprigs make better Nemelex worshipers than Mummies. The thing you want to sac more than anything is corpses (makes summoning deck gift more likely), which most sprigs can't do anything with. Potions just lead to more mostly-useless wonders decks