Mummies, since version 0.5 (and possibly earlier, but I noticed it less before) seem to be distinctly underpowered. All the penalties they've been given are presumably based on the need to prevent them from being overpowered due to their natural ability to ignore spell hunger - which is a very important concern - but the drawbacks seem to be somewhat excessive, as explained below.
1) Low starting attributes, and no automatic attribute gain on level-up. The code cites lack of hunger and "challenge" status as the reason for giving mummies and ghouls respectively the worst stats of any race, but ghouls still get slow Strength increases with experience. Mummies, which have no indication of being intended as a difficult race to play, don't. And because their high attribute is Strength, they're automatically disadvantaged as spellcasters: a first-level mummy wizard casting Magic Dart, the simplest spell in his arsenal, has only about a 75% chance of success. And since he has only five spell points and Magic Dart does 1d8 damage, it's almost guaranteed that he'll end up having to run if he accidentally alerts any group of more than two enemies. And if they're fast (like a pack of jackals, which are common on D:1), death is nearly certain.
2) Almost universally bad aptitudes, and one of the slowest species to level. Their only aptitudes that are on an average level are fighting, spellcasting, and necromancy. This would theoretically make them best suited for use as death knights, but as I'll explain later that isn't the case. (The aptitudes aren't a problem on their own, but combined with the other items of this list it becomes worse.)
3) They're the only race to start with a negative resistance. This one actually makes sense given that they're dried, kindling-wrapped corpses, but it does give them a significant early-game disadvantage. Draconians may end up with a negative resistance as well, but it only kicks in at experience level seven when they'll have a +res item or enough hit points to survive running away. (Note also that player mummies are one of the only two corporeal undead NOT resistant to cold. The other is the "flaming corpse" monster, which has a good reason not to be resistant.)
4) Inability to drink potions. This is also a perfectly reasonable drawback for a mummy, but it hurts them far more than the other penalties. It makes them nearly worthless in melee* because they have no emergency healing source unless they've found a wand of healing, which is very unlikely in the early game. They're also now prohibited from being magically healed by vampiric weapons for whatever reason. Because of this and the second half of #1 above, mummies will have a good deal of trouble succeeding at anything.
*Makhleb only helps if you're fighting many weak creatures rather than one powerful one, and weak creatures aren't much of a problem to begin with. And it seems odd to give a non-religious class a de facto dependence on one specific god.
And I'm apparently not alone in believing that mummies are weak. According to the stats on LearnDB, mummies are third from the bottom in the ranking of non-"goodplayers'" victory by species. But within both "goodplayers" (who presumably know the game better than more casual players) and overall ranking, mummies are dead last.
Not only that, but according to remarks made by the devteam as recently as a few days ago, mummies are plagued by rampant scumming. Combined with the above statistics, this means that the percentage of legitimate winners is actually much lower than shown here. It also could mean that the mummy is too weak to hold the interest of legitimate players, who are more easily discouraged than the scummers.