What is nicks opinion of the people at the party when he says that once there the guests conduct themselves according to the rules of behavior associated with amusement parks?

Nick becomes an observer rather than an active participant. Nick is repulsed by the shallow superficiality of the people at the party. THe people have no other interest other than money and the illusion of happiness rather than happiness itself. The Jazz age, at least for the filthy rich, is a world of consumption and flat characters.