In the standard premise of an innocent person coming to the big city, one of the most popular ways of showing the differences between him and the society he has entered is to display the small, etiquette-based hypocrisies he encounters. In this case, it turns out they are really nothing but artificial constructs to confuse and embarrass the newcomer. The Idiot does this quite well, but it also ventures deeper, into the hypocritical behavior under that behavior. Motivations that drive small, meaningless actions in society are perceived as suspect when taken in in larger doses by the newcomer.
It is actually Myshkin's complete inability to be a hypocrite that makes him a totally unsuitable partner for both Nastasya and Aglaya.
The characters who function most successfully in the world of the novel are those who are able to moderate their truthfulness with hypocrisy—they are actually the ones who are the most attuned to those around them.