Can you sacrifice a tapped creature?
As a general answer: Whenever you are asked by a spell or ability for a certain target, or when you have to pay certain costs, then as long as the chosen object has all the required properties, all other properties can be whatever they are. 

So if an ability requires you to "sacrifice a creature", it doesn't say anything about the creature being tapped or untapped, so as long as it's a creature, you can sacc it.