Desert is a land subtype with no special meaning. It doesn't grant the land an intrinsic mana ability. Other cards may care about which lands are Deserts.

If you sacrifice Dunes of the Dead to pay the activation cost of an ability, you'll create the Zombie token before that activated ability resolves.

Dunes of the Dead's second ability doesn't allow you to sacrifice it whenever you'd like. You must find another way to get Dunes of the Dead into the graveyard.