I watched the debate, and the instant I heard him do the "that one" remark, I told my wife that I thought it was quite un-presidential and disrespectful. I thought it was awful.
World leaders are supposed to conduct themselves with a level of dignity, diplomacy, and stature, especially among peers.
Is that how he would handle himself at peace or disarmament talks with other national leaders?
And no, it is not a really big deal, just another demerit in a long column when evaluating how the man might perform as a President.