when someone gets what they deserve, or what is coming to them, they get their "Just Desserts." because of what Nevel did, people hate on him for yelling at that girl, and so he is getting his Just Desserts. Freddie used that to make a joke. So, when asked what kind of food will be at the karma party, he says "Oh, Just Desserts" as a parody of what "Just Desserts" means.
Oh, thank you for explaining that to me. I have never heard of that before. So, thanks for telling me about it! I appreciate it!