Nick has set himself up as an observer and also as one that is supposed to be brutally honest about the other people he observes. As he describes the very shallow nature and actions of the people at Gatsby's parties, it is clear that he feels they are all inconsequential and simply people who thrive on the idea of wealth and the feeling that it makes everything beautiful and good, just as Daisy feels that money can make anything beautiful.
From his tone in description and the way he points out the idiocy of many of the revelers without giving them any great depth or character, it becomes very clear that Nick looks down on them as parasites or hangers-on who float from party to party without any real existence or substance to them.
Nick's tone is derisively mocking and sarcastic. In chapter 3 he uses metaphors devoid of any true meaning in describing the guests. The descriptors give the idea that the guests are shallow, materialistic and of no real importance. They are there only to put on a show, be pretentious and indulgent, keen to exploit Jay Gatsby's generosity for all that it is worth. He mentions, for example, that:
"the air is alive with chatter and laughter, and casual innuendo and introductions forgotten on the spot, and enthusiastic meetings between women who never knew each other’s names."
It is clear from this description that the guests are not really there to make friends, for they forget about one another as soon as they've been introduced. They are all just acting out.
Nick refers to some of the girls as "gypsies" implying that they are indigent and wayward, seeking some kind of stability and attention. The guests
" ... conducted themselves according to the rules of behavior associated with amusement parks."
They were all there just to have a good time, seeking a thrill, some form of excitement to make their lives meaningful and make them seem important. Others were there to apparently just look for opportunities to make a quick buck, whilst others only came to seek recognition. Many of the guests were not even invited, but showed up anyway.
Nick is clearly disgusted by what he sees: the wanton drunkenness, irrelevant gossip and shallow, meaningless conversations as well as the typical and hysterical "party-animal" kind of activities the guests indulge in.
Nick emphasises his derision and aversion to the guests in the names he provides in chapter 4. The names are over-the-top double-barrelled affirmations of some of the guests' so-called status or preposterous derivatives of the names of important-sounding characters. Many of the names are laughable: Chester Beckers' the "Leeches" (which has an obvious negative connotation), the "Beavers" (as in "eager beaver"), etc. Nick is clearly poking fun at these shallow and materialistic individuals, who to him are not worth much in spite of their grandiose names and mannerisms.
Nick affirms his disdain for these people in chapter eight when he takes leave of Gatsby and calls out:
“They’re a rotten crowd... You’re worth the whole damn bunch put together.”
Nick's descriptions make it clear that the Jazz Age was all about being pretentious and seeking what people believed was the ultimate pleasure through self-indulgence and partying, mostly at the expense of others. It was a shallow, base existence, a desperate desire to break away from the sorrow and deep tragedy that came in the wake of the First World War. People wanted to show everyone else that they were happy and successful, but, as Nick pertinently points out, their attempts were a dismal failure.