What instead ? Estalt ? He won't cut it and Ielena is better anyways.
Ielena rocks with her ability. And if you are able to play her with fury, you just created a total life difference of 10 hitpoints with an 8 power card. Tell me again that's not good.
I personally don't like Mojo too much. Imho Mojo get's only good with fury which is kinda predictable in that case. I would use Dieter instead.
I played in elo some matches with a nightmare/pussycats deck in different versions. although estalt has greater damage and can win against the "great" cards of the game he also can lose quite easily to any power 5 card... ielena has good "muscle power" and with fury she can make, as ibtd said, a life difference of 10, sometimes enough to win a match.
mojo also can make a life difference of 9 with fury, the reason I've put mojo is based on the idea of a deck where all cards can deal massive damage or gain lots of life points so that the opponent never know what's going to hit him.
On the other hand dieter is a decent card and can help a lot against cards that rely upon their abilities. It depends on the metagame, mojo is a bit more scary for the opponent but dieter can help in many situations.
it's all up to you to test and tweak the deck