I believe that Nick is a reliable narrator, though there are many readers who would disagree with me. He narrates the story as a first-person objective narrator, meaning that he describes events after they have already taken place. He has had time to reflect on how he would like to present these events, giving him the opportunity to color things the way he would like to—making certain characters look good or bad, whichever he prefers. He knows how everything turns out, of course, and this could change the way he portrays certain characters throughout. For example, he tells us right away how he feels about Gatsby, and he later tells Gatsby himself that he is "worth the whole damn bunch [of the rest of the characters] put together." He also comes to understand that Daisy and Tom are not good people and are both entitled and "careless"; Nick says that the couple "smashed up things and creatures and then retreated back into their money or their vast carelessness or whatever it was that kept them together, and let other people clean up the mess they had made." This opinion clearly does factor in to his portrayal of the couple, even early in the narrative.
Nick is honest when he describes himself getting drunk, even during this Prohibition Era. He is honest when he describes his feelings about Jordan Baker—even honest about her significant flaws too—and one might imagine that if he were trying to make himself look good, he would obscure these moments. However, he has become disillusioned as a result of the war and his experiences in New York, and his narrative simply reflects this disillusionment, in my view.