The excerpt that you used above is actually a put down. Nick, who is actually Jordan's boyfriend at one point in this book as well as the first person "I" narrator throughout the book, is very surprised at Jordan's put down of the two girls who were telling her they were sorry that she hadn't won her tennis match.
Of course, the put down is said at exactly at the same time that the girls move on to speak with other guests. The point is that everything about this party is superficial--even the put downs, which is very surprising to Nick (That's why he is so startled by what Jordan says.), who isn't used to this kind of phony people and behavior. Even the moon is phony--it's a premature moon, one that shows the usual appearance of a beautiful moon, but one that isn't very developed--kind of like the lack of depth in most of the characters' personalities. That's why the author refers to the moon being produced out of a caterer's basket--it is a cheap imitation of the real thing, just like every person at that party is a cheap imitation of real people with real problems who don't have lots of money to help them get by in the world.
I hope that helps.