I want to get Datatag like this {EntityTag:{id:"Creeper"}}

is there a way i can get an items DataTag?
like this Spawner eggs {EntityTag:{id:“Creeper”}}

Sponge uses the Data API for this. In this case, you probably want itemStack.get(Keys.SPAWNABLE_ENTITY_TYPE).get(). Be aware that if the item isn’t a spawn egg, the final .get() call will throw an exception.

@JBYoshi Keys.SPAWNABLE_ENTITY_TYPE was designed before eggs used NBT and so only supports EntityType. So it’ll work fine for Sponge 4.1.0, but probably not for 1.9 (unless MC has built in checks for setting the item damage in 1.9)