There is absolutely no reason why Steelskull would outdamage Narma. Without getting more info it's hard to know exactly why you're experiencing this, but Narma has higher multipliers on all of her attacks AND brings poisons. There must be other factors at play that you're not mentioning.

Unbooked Narma has a six turn cooldown between her posions from the A3, and only a 75% chance for landing them. A booked Steelskull just puts out more poison. 

Probably a booked Narma would win the duel, the cooldown decrease from 6 to 4 turns is massive, and 100% chance is a lot better than 75% for landing the debuffs.

On the other hand, I don't think the difference is worth 16 legendary books if you allready have a working team. Narma's big advantage is, that she can take two jobs at once, decr. attack and poison. Her decr. attack can be booked to 3 turn cooldown, lasting 2 turns, that is good enough, as the stun-hit from CB is not atk based, so 1 turn without debuff doesn't harm you (has to be the right turn, of course). If your team allready has a atk debuffer, Narma can't shine with her full power, her atk debuff potential is wasted.

You're forgetting that Narma's A1 is a debuff extender as well. That's a pretty huge factor, especially if you're running CA.

I do not use an uk team and I do not have god-like artifacts so multipliers are not relevant to me. Narma's damage is ATK based so it makes no sense to me to skill/equip her like a dd at CB.

BTW: does the game shows anything about multipliers? I can't see any, sorry.

You're right, an unbooked Narma cannot reveal her full potential, but would a booked Narma really outnumber Steelskull by damage by far??? That is the question! What is the value of 16 legendary books measured in damage in a CB fight? After booking Narma she must improve her damage at least by 5 million to be as good as Steelskull. But of course, that's not enough.

If I could use her as an ATK down debuffer instead of Altan then I would need another champ which could outnumber Altan by damage. But then I would loose my continuous DEF increase buff (Martyr and Altan).