Sammy is fairly well-developed as a character. He is, after all, the protagonist, so his internal emotions and struggles are developed particularly well. We can also surmise a few things about Sammy's background that aren't directly stated based on his reaction to the girls who wander through the A&P on his shift.
Part of Sammy's internal struggle is that he longs for something else. In Queenie and her friends, Sammy recognizes a freedom that he himself lacks. They wander the store with confidence, disregarding the social norms of the store. Sammy contrasts them with the "sheep" who push their buggies through the aisles, whose paths of conformance are interrupted by these girls "walking against the usual traffic," yet they snap their eyes back to their own baskets. Through his observations, we know that Sammy longs to live in the carefree and privileged world of Queenie and not align himself with people like Stokesie, whom Sammy notes already has "two babies chalked up on his fuselage already," even though he is barely older than Sammy.
We also see the immaturity in Sammy's observations. He is keenly (perhaps overly so) aware of the beauty of the girls, noting every detail from the texture of their swimsuits to the "bare plane of the top of her chest down from the shoulder bones." He impulsively decides to quit his job in an intended grand romantic gesture that he hopes will capture the attention of Queenie and impress her, allowing a path for Sammy to enter her world. Yet his grand gesture falls flat, and it is only then that Sammy begins to realize some truth about his situation.
The author leaves Sammy's character growth—or lack thereof—greatly unknown, so we aren't sure if he really does transform as a result of this conflict. We know that his parents are depending on him to keep this job, and we know that his mother had ironed his work shirt for him the night before. Thus, Sammy is fairly reliant on his parents and their support, even at nineteen. Will this event change him? It's hard to say, and that final line can imply various outcomes for Sammy.
I would define a hero as someone whose actions place the needs of others above the self. I don't think Sammy's reaction falls into that category. Ultimately, I think his attempt to impress the girls is self-serving; he wants Queenie to notice him and therefore gain access to her world. This isn't heroic but selfish.
The physician in "Godfather Death" has similar character flaws. When given the opportunity to become king and marry a beautiful princess, he is willing to defy the laws of natural order in order to achieve his own ambitions. Depending on which version you read, he is also willing to sacrifice the life of a child in order to do so. Sammy's character is more developed because it is written from a first-person point of view, with more space allotted to his inner thoughts and conflicts.