There are differing opinions about this. I don't see a problem with it: Your total check is less than if you were getting a full meal, but you're occupying space in the restaurant for a much shorter period of time, so it seems all-fair to me. If you have any doubts, just make it clear when you request your reservation: "I would like to make a reservation for dessert at XXX restaurant, on YYYY date, at ZZZZ time."