The novel "Catcher In the Rye" by J D Salinger is seen as a serious novel because it deals woith pretty serious and heavy themes - death,bereavement,nervous breakdown,mental health and the welfare of young people. Although Holden Caulfield himself uses some casual slang, bad language and a sarcastic humor, the actual things he says, and his issues are anything but funny. They are quite sad and desperate. He seems a lonely, isolated figure alienated from everyone that could help him including his own family. His view on society and school seems bitter and cynical - he sees these people as being "phony" or "fake." Most parents faced with a cry for help like this from their teen son would see the situation as being pretty serious.
Despite it's clear ironic humor, how is Catcher in the Rye NOT a serious novel? It epitomizes the tragic rebellion and ultimate depression of an overprivledged teenager who has finally hit the ceiling on his dislike for "the system."
After working in both public and private schools, I've seen that Holden Caulfield is not entirely unique (a fact which, had he realized it, might have helped him). He certainly isn't common, but too many kids stuck in a private education feel the pressure of school administrators, parents, and peers for so long, they forget, or worse, never come to know their own identities.
This is Holden Caulfield in a nutshell. While the book is written from his own sardonic view of his world, the seriousness lies in the truth behind his emotions, which are largely due to an upbringing he neither asked for, nor had any support in.
I would say that the novel is quite serious. At the time of writing, it explored issues and presented contexts that we now take as common. Yet, for the time period, it was shockingly honest and very serious in terms of how it approached the individual and a relationship to society. The way in which inauthenticity is depicted in both institutions, social interactions, and personal interactions are all quite serious. Holden does a good job in exposing the "phonies" that are present in his own life, and in the process, in the world. At the same time, I would say that the redemptive relationship between Holden and Phoebe is very powerful and very serious. I think that being able to find this one relationship amidst a world that denies significant emotional contact is highly effective.
There are hundreds of answers to this question; one answer might include the fact that the book was banned from many high schools for use as curriculum, due to its “explicit” content