To me, the major theme of this story is individuality. We see, in this story, how people are treated when they act as individuals.
In both the case of the girls and the case of Sammy, acting as an individual does not do a person much good.
The girls, you can argue, are expressing individuality and rebellion by going to the store in bathing suits. For doing this, they get scolded and are a bit of a spectacle.
Sammy expresses his individuality by standing up to his boss who is clearly in the right (by the standards of the time). He gets fired.
This is a humorous, yet poignant coming-of-age story. It is a story of self-discovery, an initiation story where a teen crosses a threshold into manhood--and regrets it.
There are only a few ways a boy can become a man, and most of them deal with rebellion against a same-sex authority figure. Updike's story is no different: his narrator tries to break away from the conservative materialism of the adult world, only to discover that no one takes notice--and he's left without a job. Worse, he knows now that fighting against adulthood is a losing cause, morally and financially.
The perfect theme is reflected in the last lines:
I look around for my girls, but they're gone, of course. There wasn't anybody but some young married screaming with her children about some candy they didn't get by the door of a powder-blue Falcon station wagon. Looking back in the big windows, over the bags of peat moss and aluminum lawn furniture stacked on the pavement, I could see Lengel in my place in the slot, checking the sheep through. His face was dark gray and his back stiff, as if he'djust had an injection of iron, and my stomach kind of fell as I felt how hard the world was going to be to me hereafter.
The speaker knows he's acted foolishly, idealistically, even romantically. He thought the world would stop spinning when he quit. He thought at least he'd get a phone number. But, alas, he's sabotaged himself--for what? Girls? They're gone. To spite Lengel? He'll find someone else. For principles? What were they, anyway?