The "negative" in this text means a "No" vote on Scottish independence. To "dress up a negative" is to emphasize the good qualities of what is otherwise a negative or unpleasant thing. The author is saying that it is hard to cast the "No" result of the referendum as anything other than a rejection of the argument for independence; in other words, it's just a "No" and there's not much that one can do to take the sting out of it.

"dress up" means "make more attractive", like when you wear ("dress in") your best clothes to impress someone. The text is saying that it is difficult to make something negative seem more positive.