What is the main theme of "A Supermarket in California" by Allen Ginsberg?

In "A Supermarket in California," Allen Ginsberg movingly contrasts themes of connection and isolation as he imagines encountering the poet Walt Whitman in a fruit market late at night. Ginsberg pays homage to the expansiveness and sensuality of Whitman's poetry, but recognizes the limitations of this legacy in his own life.
In the opening of the poem, the speaker is "shopping for images," or seeking inspiration for writing. A late-night fruit market provides the opportunity to reflect on and imitate the qualities of Walt Whitman's poetry, which famously embraces sensuality and unites the experiences of a world full of people. Ginsberg uses lists of people and groceries to create a full, lively landscape. His descriptions are playful, placing "Wives in the avocados, babies in the tomatoes!" in a hyperbolic moment of excess.
At this point, the supermarket becomes a place where the literary legacy of Walt Whitman is physically manifested. The scene even includes a call out to the poet Federico Garcia Lorca, another poet for whom sensuality was important and who published his own "Ode to Walt Whitman" in 1955. The speaker of the poem describes Whitman as "childless" and "eyeing the grocery boys," referencing the older poet's homoerotic work and connecting it with his own life as a gay writer. The two poets connect, the speaker following Whitman "in and out of the brilliant stacks of cans," enacting the homage he is paying by writing this poem.
This moment of connection is complicated by the presence of danger. The speaker imagines himself followed "by the store detective," a force of law threatening the bacchanalian "odyssey in the supermarket." He begins to feel lost and uncertain, asking "Where are we going, Walt Whitman?" He concludes that no matter where they go, they will "both be lonely," separated both by Whitman's death and by a larger cultural lack of connection.
The speaker acknowledges that, in writing this poem to Whitman, he is "dreaming of the lost America of love." He exists in a country dominated by consumerism (represented by the cashier, the supermarket full of goods, and the "blue automobiles in driveways"), and law (represented by the store detective). Despite the force and joy of Whitman's legacy, life and death both leave us lonely.

Wishing, as he says, "self-conscious[ly]" to find a list of images similar to those Walt Whitman used in Leaves of Grass, the speaker wanders into a modern supermarket. There he finds the hallmarks of modern American materialism: neon lights, families shopping at night, peaches, avocados, and tomatoes.
The theme is that this supermarket is a debasement of the America Whitman described a hundred years earlier. Consumerism has taken over. In contrast to a world ruled by money, the speaker imagines he and Whitman strolling through the store and liberally sampling the wares without worrying about paying:

tasting artichokes, possessing every frozen delicacy, and never passing the cashier

It seems, the speaker says, "absurd" to put together Whitman and this supermarket. He imagines instead walking with Whitman, as they

stroll dreaming of the lost America of love past blue automobiles ...

However, while contrasting Whitman's "lost America of love" to his own culture, the speaker also weaves in the theme of loneliness. He uses the word "lonely" three times, twice to describe Whitman and once to describe the loneliness he and Whitman share. Both, whether writing about the United States a century ago or today, feel apart, at least in the speaker's imagination. That Ginsberg is alluding to Whitman's being gay when he says he is "lonely" is suggested in the line that he is "childless" and "eyeing the grocery boys."
Although Whitman exuberantly celebrated his oneness with all of America and the entire universe, the speaker is orienting both himself and Whitman as outside of the mainstream of American life. They are not part of a world of supermarkets filled with shopping heterosexual families.

Beat writers like Allen Ginsberg and Jack Kerouac often focused on Americana in their works. The Beat Generation was understood as a generation lost; the Beat poets lived in the shadow of World War II, and instead they had time to think about what it meant to be American in the absence of romanticized heroic conflict. The Beat poets wanted to find the heart of what their generation meant in the center of the twentieth century. With this, many of Ginsberg’s (as well as other Beat poets's) themes repeat themselves.
In Ginsberg’s “A Supermarket in California,” Ginsberg is recording what he thought America had become. He writes, “In my hungry fatigue, and shopping for images, I went into the neon fruit supermarket, dreaming of your enumerations!” Ginsberg is essentially saying that he is fatigued by the barrage of American culture, especially America’s need of romance and consumerism. The idea of “shopping for images,” is reminiscent of Plato’s “Allegory of the Cave,” as Ginsberg is not shopping for a specific item, but an image that is placed before him that is shadowing reality.
Ginsberg calls out to Walt Whitman many times in the poem, questioning what he would have done if he were alive in the 1950s. In Ginsberg’s anxiety of the future he writes, “Where are we going, Walt Whitman? The doors close in an hour. Which way does your beard point tonight?” Ginsberg calls on Whitman for advice and asks the most basic question, “where are we going?” While the Beats have no great war to fight and are trying to identify what it means to be a free individual in the United States, the rest of the country is doing the same—lost without a sense of direction.

Ginsberg's poem focuses on commercialism and its spread in the economic boom following World War II. The title indicates the centrality of the supermarket, a twentieth-century American invention that signifies the expansion of commercialism. Supermarkets expanded the variety of items that could be purchased from one store, encouraging easy, cheap—and therefore likely more excessive—buying. The setting is California, where Ginsberg lived and taught for many years, a place that had once captured his poetic imagination as a place of natural wonder. His poem engages in this setting with curiosity.
As he explores commercialism, he also looks at the theme of vicarious identity. He places Walt Whitman and Federico García Lorca, poets to whom he relates and whom he admires, in a place they never knew. He is exploring how they would be as he wonders how he should be (all three are also gay writers specifically). Ginsberg's poem marks a specific moment in American history, recording international anxieties, sexual repression, and rampant American capitalism all simultaneously.

There are several possible themes in Allen Ginsberg's "A Supermarket in California," but the most prominent are the ideas of inherited literary influence and legacy, sexual identity, loneliness, and American consumerism and commodification.
The narrator begins the poem by saying "What thoughts I have of you, tonight, Walt Whitman" (line 1). In the poem, the narrator thinks abut Whitman, imagines he sees him, and has an internal dialogue with him throughout the poem. He also imagines that he sees Garcia Lorca. Both of these poets were influential for Ginsberg, and "A Supermarket in California" imitates Whitman's style and updates his themes and subject matter. Throughout the poem, although the narrator is alone, he imagines being together with Whitman in "our odyssey" (line 18). Throughout the text the narrator acknowledges the literary debt that he has to Whitman in terms of inspiration and style.
Also, it is a common conception that Whitman was homosexual. The narrator alludes to this idea in the second stanza of his poem when he says "I saw you, Walt Whitman, childless, lonely old grubber, poking among the meats in the refrigerator and eyeing grocery boys" (lines 8-9). In his lonely journey through the store, this is another factor that aligns the narrator with his imagined companions.
Throughout the poem there is a sense of sadness and loneliness. The narrator feels "absurd" (line 18), "self-conscious" (line 2) and "lonely" (lines 20 and 23) as he wanders through the supermarket and dreams of his literary inspirations.
Finally, there is a sense of critique about American consumerism and commodification. Whitman lived in the mid-1800s, when America was rapidly industrializing, but most of his poetry focuses on the natural world and the beauty that can be found there. By contrast, this narrator wanders through a busy "neon fruit supermarket" (line 3) and feels lonely in the midst of all the bustling people. In his final stanza of the text the narrator wonders what America Whitman used to have before it all disappeared.
https://www.poetryfoundation.org/poems/47660/a-supermarket-in-california