The short answer is that Fitzgerald does use a stereotype to describe Nick: The reasonably well-to-do young man come east to "make his fortune." He is genteel, conservative, rather aimless, and financially independent. He is, however, much more than a mere stereotype; as the main character in the novel, he possesses more depth. This part of your question is worthy of a major essay by itself.
Nick is really the only dynamic character in the novel. Gatsby remains true to his dream; Daisy, Tom and Jordan continue in their careless lifestyles; even George Wilson remains unchanged -- he is weak and easily deceived. Nick, however, begins by admiring the glittering, wealthy people of East Egg, but becomes more and more disillusioned with them as the book progresses. By the end of the book he sees Daisy and Tom as little more than children, who carelessly break things and make messes, but move on, leaving others to clean up after them. He came to the East in order to make his fortune and become one of the wealthy elite; he ends up leaving the East for the West, which is presented as pure and honest throughout the novel.