"A Supermarket in California" doesn't use the word "America" until the end of the poem, but that doesn't mean this one's not all about our fair nation. Ginsberg imagines an America that fits a very 1950s ideal: blue automobiles in driveways of suburban homes, whole families shopping together. The speaker feels like an outsider in this America, which is all about the things you can buy; he conjures up Whitman who, he hopes, represents a "lost America of love," which was more about love than about things. But in the last line of the poem, the speaker calls all this into question: was there really ever an "America of love"? Or, like Walt Whitman, is this all a fantasy?
"A Supermarket in California" is a critique of 1950s American consumer culture, and Ginsberg wishes that he could get back to the good old days before people went "shopping for images."
"A Supermarket in California" is a critique of 1950s American consumer culture specifically, but Ginsberg knows that America has always been this way. There's no such thing as a "lost America of love."