The significance of Nick's statement relates to social attitudes toward wealth. The "old money" view the "new money's" display of wealth as vulgar and ostentatious. Gatsby's parties are a fine representation of that, with all the excesses and lack of discernment (even obliviousness) in the majority who attend. Of the variety of ways in which Fitzgerald illustrates this point throughout the book, the references to the homes of Gatsby and Tom Buchanan, and their juxtaposition, is perhaps the most vivid.
Nick says to Gatsby: " Your place looks like the world's fair." What is significant about this statement? What else might Nick mean?
Well, we can't know what ELSE Nick might mean, because it seems to be an ambiguous statement: all multiple meanings are right there in the sentence.
On one hand, it means that that Gatsby's place looks bright. It is all lit up, and looks impressive. On the other hand, it means that it looks like a fair: too many people have access, and up close there's lots of trash and waste, etc.
It means both at once.