I don't know that it's frowned up; there are no rules stating you have to order a certain amount of food but I see a problem in the logistics of it. Most people make their ADR's at least 90 minutes before fireworks in order to be seated when the show starts. People who are finished 30 minutes before the show aren't going to vacate their seats. So incoming diners start stacking up waiting on tables. We've waited up to an hour past our ADR time to be seated here.
Since you just want dessert, which takes all of 15 minutes to be served, you wouldn't want an ADR time 90 or even 60 minutes prior to fireworks. Anything later than that, you may not even be seated prior to fireworks.
They will let diners back up with their receipts so you might want to consider going early, having your dessert and then returning to the restaurant with your receipt at the time of fireworks.