I don't think it would be appropriate.
CG seems to be booked solidly every night. If you go just for dessert, you would be taking a table from a family who actually wants to dine there....and dining is the main point of the restaurant.
It would also impact your server. Instead of earning a tip on a meal, it would be a much smaller tip for just dessert.
Just going up won't work either. Apparently too many people were doing that so now they control access. You need to have a reservation. However, folks have reported that they do allow guests up to order a drink at the bar if there is space. I sort of doubt there's much space near the time of Wishes though.
There is a walkout on the 4th floor where you could enjoy Wishes. But it truly best from inside the park as there are aspects of it that can't be seen from anywhere else.