Another (much less apropos) reason that that wouldn't work is that normally, it takes black gems worth 75 gp per HD (I believe) to create a zombie or skeleton. However, it may be that there's some homebrew or dedicated rule-forgetting in this world that lets you go without the material components (we've seen several instances of people animating dead without any sort of gems, but then, V never uses material components in her spells either; perhaps the gp is taken into account off-screen?)

Anyways, if you're high enough level to destroy cities with impunity, a crapload of one HD commoner zombies are going to do more to get into your way than to help you. I'm assuming that in Xykon's army, the number of possible undead was maxed-out, or nearly so, with all the clerics and Xykon putting a lot of effort into making and maintaining them. Further more, unless you're Ganonron, you can't teleport an army.