On the first pages of the novel, Nick says that his father advised him not to judge people. However, his judgements are present throughout as he falls in with Gatsby and his circle and his cousin Daisy and hers. Towardst the beginning of the book Nick seems to judge Gatsby as a fake. He notices that Gatsby uses a stilted and purposeful manner of speech that is the hallmark of someone trying too hard. When he talks about his heroics in the war, he quickly pulls out a medal awarded to him by the country of Montenegro. When he talks about this time at Oxford, he presents a photograph as proof. Nick sees through Gatsby's facade.
Nick does, however, act as a go-between in Gatsby's campaign to win Daisy back. But after the revelation of their affair and Myrtle's untimely death, Nick realizes that it's really Tom and Daisy who are the phonies. They are "careless" people who enter and ruin the lives of their innocent victims. The last time Nick sees Gatsby before his death he says that Gatsby is better than the "whole lot" of the others.
Nick's realization and inability to reserve judgement are evidence of a profound change of character.