Why is the issue of judgment and morality so important in the novel?
The Great Gatsby is a novel about perception as much as it is about any other themes. Perception of social standing, of moral standing, and even perception of self are each explored in some depth.
Each of these modes of perception involves judgement of some kind. Social standing is an intrinsically relative concept: In order to determine where a person exists in the social order, you have to judge who is above and who is below. In a book so concerned with social climbing, this kind judgement is unavoidable.
The latter modes of perception are more subtle and also much more anchored in morality.
Questions arise as to the morality of Daisy's plan to run away with Gatsby. Is it morally acceptable for her to cheat on Tom if he is already cheating on her? Is it morally acceptable for Daisy to leave Tom simply because she "truly loves" Gatsby? (And is Gatsby acting immorally in courting a married woman?) Should Gatsby see himself as a fraud and a transgressor? Can he see himself in this way?
The connecting thread between each of these questions is a moral one relating to a fundamental honesty. Ultimately, these questions remain open to interpretation, yet they focus the narrative - as presented by Nick - and they help to ask the larger questions as to...
1) How much anyone should be expected to be honest in (marriage, in a social setting)?
2) How honest are people actually capable of being?
These questions are directly related to morality in general and are impossible to imagine without a framework of discernment and judgement.