Much like the style, the tone of the novel is the tone of Holden Caulfield, since he's the one telling the story. We don't think we need to spin you a large defense for our use of adjectives such as "cynical" and "judgmental" – just pick a page. But it seems we have some explaining to do; how can the tone be both cynical and compassionate? Check out Holden's interaction with Sunny; his digression about her buying the green dress shows incredible compassion on his part. What depresses him is the thought that she is a young and nervous person – not unlike himself – who is stuck in a not-so-pleasant role in life.
The Catcher in the Rye is full of this sort of observant compassion, and its tone is in part defined by it. As for the humor, it's definitely there – like Holden's claim that if Ossenburger were to pray, it'd probably be for Jesus to "send him a few more stiffs."
Of course, "sad" doesn't need much of a defense either. Implicitly, Holden finds nearly everything to be depressing. For example, when he actually uses the word "depressing," he names the following: leaving a place without a proper good-bye, leaving a place with a proper good-bye, men in bathrobes, Vicks nose drops, Old Spencer's warnings, people who say "good luck," empty corridors in the dorm, getting ice skates as a present, getting presents in general, girls that travel to New York from Seattle to wear ugly hats and see the first show at Radio City Music Hall, hotel lobbies, people laughing on New York City streets late at night, prostitute's dresses, having money, not having money, inexpensive suitcases, money in general, nuns and their lack of swanky lunches, girls and their potential futures, being drunk, being told he doesn't like anything, sleeping in train stations (although, to be fair, this is objectively depressing), the fact that Mr. Antolini made a pass at him, the fact that Mr. Antolini might have not actually made a pass at him after all, vulgar wall graffiti, the fact that he couldn't take revenge on the person who wrote said graffiti even if given the opportunity, Phoebe or people like her saying the word "please," and the fact that he has told us any of this at all.
But the most interesting (and most illuminating) aspect of the tone to discuss is the one that Holden comments on within the text itself – digressions. Check out his discussion with Mr. Antolini (before the big education lecture) about the Oral Expression class at Pencey. Holden failed the class because he felt digressions were important. He makes a few key statements: 1) "I like it when somebody digresses. It's more interesting and all." 2) "Lots of times you don't know what interests you most till you start talking about something that doesn't interest you most." 3) "You can't hardly ever simplify and unify something just because somebody wants you to."
Wait a second – Holden's talking about an Oral Expression class. What's that got to do with his story? A lot. He's telling us his story. He's not writing it down and handing it over as an essay, he's verbally telling it to us, and as you can see from the surrounding discussions on style and tone, he's gone to great lengths to make it sound not like written exposition, but like verbal communication, or oral expression, as it were.
So that means everything he just said to Mr. Antolini, he's actually saying to us. He's not devoting three pages to a random discussion on oral style for nothing; he's defending his narrative technique. Why is Holden telling us about Allie's baseball mitt? About the time he played checkers with Jane? About James Castle jumping out the window? Because 1) it's more interesting, 2) he didn't know that was what he needed to talk about until he started, and 3) he couldn't simplify and unify these events just because that would make it easier for us.