If Mike Huckabee wants to spend his campaign money to wish voters a Merry Christmas there is nothing to stop him.
If he wants to appeal, subliminally or overtly, to Christian voters there is no reason he shouldn't.
Unless and until he says that he's the best qualified simply because of his faith or that it should be the single deciding factor there is no good compelling reason to tell him to knock it off.
Find something else to be upset about. There's plenty out there.