Amir grows up to be a writer, and like all writers, he deviates from what are considered "correct" rules and norms of the language. His comment on clichés is a good example of this deviation; most writing books and teachers will tell growing writers to avoid clichés simply because of their nature. A cliché is, by definition, something that is commonly used or overused "in such a way that it loses its original meaning or effect" (Wikipedia) and usually when people see clichés, they assume that the work as a whole will follow this path of being just like everything else in its genre. For example, the common film trope of "Die Hard on an (X)" follows the overused plot of the film Die Hard but changes the venue, causing the viewer to associate the work with minimal effort or caring.
A creative writing teacher at San Jose State used to say about clichés: "Avoid them like the plague." Then he'd laugh at his own joke... I always thought clichés got a bum rap. Because, often, they're dead-on. But the aptness of the clichéd saying is overshadowed by the nature of the saying as a cliché.
(Hosseini, The Kite Runner, Google Books)
Amir's comment on clichés might be interpreted as a need to have common points of reference; while the cliché itself might be overused, there are often no other ways to describe a situation. Amir goes on to mention "The elephant in the room" to describe his reunion with Rahim Khan, explaining that the concept of a large, unavoidable subject that is nonetheless unmentioned perfectly describes the meeting. In certain cases, then, it is not only appropriate to use clichés, but it becomes almost impossible to not use them. Amir's comment thus becomes both a denunciation of keeping only to accepted writing principles -- such as avoiding clichés -- and a comment on the tendency of rules to constrain art.