While the poem may be called "A Supermarket in California," only about half of the action in the poem takes place in the "neon" food store, where there are families, old ghostly poets, and lots of fruit and vegetables filling the scene. The poem actually begins and ends outside on the street.
It's nighttime, and its dark, and the speaker wanders aimlessly, staring at the full moon and posing deep questions about the meaning of life that he'll never get answers to.
There is an extreme contrast, then, between the fluorescent lights of the supermarket and the deep darkness of the night. This contrast is played out in the poem as well; the poem features some true moments of happiness—wives in the avocados, babies in the tomatoes. But it also has some despair in all that lonely moon watching.
It's also worth noting that this poem is set in California. Ginsberg makes a point to put the supermarket there, rather than in Indiana or Idaho, or Iowa, or any other place that starts with an I anyhow. This choice reminds us that this poem is at least a little bit autobiographical, and also that Ginsberg was a way-famous member of the Beat Generation, who found their footing and flourished on the lonely, moonlit streets of the City by the Bay.