I think it is important to distinguish between the way Holden interacts with other characters in the novel and the way he interacts with the reader. He admits that he often lies to other people and that he often pretends to have feelings and beliefs he doesn't really have. But he is always honest with the reader even when he is describing his own dishonesty.
In fact, Holden Caulfield, as the narrator of The Catcher in the Rye, is one of the most honest and one of the most self-revealing characters in all of literature. It would be very hard to think of other characters who were more honest than Holden, or to think of other characters who were equally honest in revealing what they really think and feel. One of the few that come to mind is the anonymous narrator of Fyodor Dostoevsky's novel Notes from Underground.
The Catcher in the Rye has often been compared to Mark Twain's Huckleberry Finn. Both Huck Finn and Holden Caulfield describe the characters they encounter with the uncompromising integrity only possessed by children, who haven't yet learned to think one thing and say something different. They can't help seeing the phoniness of adults, because adults have had to become phony in order to prosper, or at least survive, in the dog-eat-dog struggle for existence.
Another character who has the untarnished vision of childhood is one who is not nearly as well known as Holden or Huck. She is a young girl named Amy in one of John Cheever's short stories, "The Sorrows of Gin." There is no better representation of the way intelligent children see adults than this passage in Cheever's story:
The voices woke Amy, and, lying in her bed, she perceived vaguely the pitiful corruption of the adult world; how crude and frail it was, like a piece of worn burlap, patched with stupidities and mistakes, useless and ugly, and yet they never saw its worthlessness, and when you pointed it out to them, they were indignant.
In the opening paragraph of Chapter 3 of The Catcher in the Rye, Holden writes:
I’m the most terrific liar you ever saw in your life. It’s awful. If I’m on my way to the store to buy a magazine, even, and somebody asks me where I’m going, I’m liable to say I’m going to the opera. It’s terrible. So when I told old Spencer I had to go to the gym to get my equipment, that was a sheer lie. I don’t even keep my goddam equipment in the gym.
Holden tells lies throughout the book—but he never lies to the reader. A good example of how Holden lies and admits he is lying is in Chapter 12 when he confesses that he cannot go through with the act he has committed himself to perform with the young prostitute Sunny.
“Nothing’s the matter.” Boy, was I getting nervous. “The thing is, I had an operation very recently.”
“On my wuddayacallit--my clavichord.”
A clavichord is something like an old-fashioned piano. Holden could have come up with a better lie than that if he hadn't been nervous and under extreme pressure.
A phony is a liar who doesn’t know he is lying. Since Holden, like Huck Finn, lies on principle, and often in self-defense, knowing he is lying and admitting he is lying, it seems unfair to call Holden phony. He isn't phony with the reader, and he isn't phony with himself.