After Stradlater asks Holden to write his essay, Holden says, “[Stradlater] wanted you to think that the only reason he was lousy at writing compositions was because he stuck all the commas in the wrong place.” Then Holden says aboutan experience with Ackley at a basketball game, “Ackley kept saying . . . that Coyle had a perfect build for basketball. God, how I hate that stuff.” Why does Holden hate this “stuff”? How does this relate to Holden’s disgust with phoniness?
The whole episode involving Holden's writing an essay for Stradlater is just one of the ways in which J. D. Salinger establishes that Holden is an exceptionally good writer for his age. This has to be impressed upon the reader in order to make it credible that Holden Caulfield, not J. D. Salinger, is the author of The Catcher in the Rye. At the same time, the quarrel with Stradlater helps to explain why a person with Holden's intelligence and verbal skills should be flunking out of school. He only writes when he feels like writing. He is entirely lacking in self-discipline. This is understandable, since he is only sixteen years old and is flunking out of his third prep school. Even though he flunked out of two other schools, he probably received "social promotion" and is now a junior, although he has had hardly any high-school-level education at all. It is probably because of Stradlater's derogatory way of asking Holden to ghost-write a paper for him that Holden ends up writing one that Stradlater can't use. In other words, it is a form of passive aggression, one of the traits that makes Holden disliked and "ostracized" by most of his fellow students. This is what prompts his roommate to say:
"You always do everything backasswards." He looked at me. "No wonder you're flunking the hell out of here," he said. "You don't do one damn thing the way you're supposed to. I mean it. Not one damn thing."
Stradlater seems to take it for granted that Holden was supposed to help him cheat. He didn't want to admit that Holden was a better writer and by implication acknowledge that Holden was more intelligent. Stradlater wanted to think he could write as good an essay as Holden if he wasn't pressed for time and if he didn't have a little trouble with trivial matters like punctuation.
Holden is already angry with Stradlater for having a date with Jane Gallagher, the one girl Holden really cares about. Then he feels offended by the left-handed way in which Stradlater asks him to ghost-write his paper. It is probably inevitable that the paper Holden produces will be defective from Stradlater's perspective, just as the fight is probably inevitable from the time Holden learns that the predatory Stradlater has Jane Gallagher waiting for him in the car.
Holden is lonely, unhappy, and angry at the world. This is why everybody looks like a phony to him--or why he only sees the phony parts of most other people. His mood colors his whole world.
His poor performance in school notwithstanding, Holden Caulfield has an extremely keen mind. For, he quickly discerns pretentiousness, insincerity, and shallowness in the words of others. Furthermore, in Chapter 4 the remarks of Stradlater and Ackley are, in effect, compensatory for their shortcomings; Stradlater has greater faults in his writing than punctuation and Ackley is probably unathletic with his sinus trouble and apparent clumsiness; certainly, he is as unpopular as an athlete is popular. So, in disparaging the basketball player by attributing his athletic success simply to his natural build, Stradlater makes himself feel better. Likewise, by attributing his poor grades on compositions to his misplacement of commas, Stradlater hides his real inadequacy and also feels better about his ability than he should in reality. Therefore, in Holden's mind, the words of Stradlater and Ackley are hypocritical ("phony") because the boys try to mask their shortcomings by diverting Holden's attention to something else. Holden disdains their insincerity and can only find comfort in the precocious innocence of his little sister, Phoebe.