Yes, you are on the right track in your thinking about these characters in the short story "A&P" by Updike. Even though Sammy doesn't seem too complicated at first and unlikely to change, he makes the decision to quit his job after Lengel, the store manager, embarrasses Queenie and the girls. Queenie and her friends represent freedom to Sammy; they come into the grocery wearing bathing suits and are barefoot. This is a grocery that's "five miles from a beach," where most people "haven't seen the ocean for twenty years." The girls flout the rules of what is expected by society.

By quitting his job, Sammy also flouts the rules because his parents and Lengel expect him to work. When Sammy decides to "pull the bow at the back of my apron," he feels a sense of manhood and freedom. He is a round character in his moment of epiphany at the end of the story when his "stomach kind of fell as I felt how hard the world was going to be to me hereafter." Through this rite of passage, Sammy is on his way to making his own decisions and taking responsibility for those decisions.

We never get to know much about Queenie besides the fact she wears a bathing suit into the store and blushes when Lengel embarrasses her, so she is a flat character, without change.

In "A & P" by John Updike, Sammy is the protagonist. The story is told from Sammy's point of view, and we find out much more about Sammy than we do about the other characters. Thus you are absolutely correct in saying that Sammy is a rounded character and the other characters are flat. This effect is strengthened by the nature of Sammy as a narrator. He is a self-centered adolescent, who has little respect or understanding for the customers at the store, as we see in the beginning of the story when he is serving an older woman who carefully watches him ringing up her purchases. Rather than understand that she may well be someone for whom every penny counts and sympathizing with her anxieties, he is rude and contemptuous to her because she is not young and pretty and scantily dressed, unlike the girls on whom he is focused.

This flattening effect also occurs as our Sammy observes Queenie and her friends, mainly through the lens of his emerging sexuality, in an example of what literary critics sometimes call the "gaze". Although as readers we can look beyond Sammy's preoccupations and see that the other characters in the story are people in their own right, Sammy as a narrator only sees them as they impinge on his own needs and desires, thus making them into flat characters in the story.