Something similar has been suggested... but not quite like this. I like the idea, it's not needed, but it's nice. I think if you go with dead girls their looks are going to have to be dropped dramatically due to the obvious. Think of it like pet cemetary, where they come back but they're nothing like they originally were.
All in all, if we're going to bring a girl back from the dead, there needs to be more cons than there are pros. So yeah, you can get some of your favorite girls back but at a cost. Also need some sort of icon, color, or simply just saying "Zombie" by the girls name to represent this.
Possible pros:
Tire slower
Tougher
High Obedience (hard to lower)
Necro fetish (got some freak customers out there)
Needs nothing more than a cage as a room (and restraints)
Cons
May randomly kill a guard when clicking next week (5% chance)
May randomly kill a customer (5% chance)
Looks lowered A LOT. (By 50% of the girls original value?)
Skills never improve or improve much slower than normal girls
MUST remain restrained (Could be a mandatory equippable or employ a necromancer to keep watch of a certain number say 5 (Could be skill based low only 1, mid 2-3 ect.) any more will require another necromancer)
Probably paid less, but can work more.
Immune to consumables
I'm personally not a fan of this one...it runs far too close to necrophilia for my tastes. From a practical standpoint, I don't know (nor do I really care to find out) if there is enough zombie girl hentai out there to make it work.
Who said you needed zombie girls to actually make it work? I suppose if you want to add randoms to it you may want that, but if you just use it to bring back girls you killed it's not really necessary.