Much of the poem takes place in a supermarket in California, so it's not like the title comes out of left field. But still, Shmoop wants to go deeper.
The poem could be called, say, "Walt Whitman, My Imaginary Friend." That would make a lot of sense to us, too. But with "A Supermarket in California," Ginsberg insists in his title that location is important, and it seems to us that there's nothing more American than a neon supermarket in California, filled with mothers, babies, and canned soup. This is not just a poem about Walt Whitman, Ginsberg, or even a supermarket. The title announces that it's a poem about America.
Plus, if we zero in even further, and focus on the fact that Ginsberg sets this poem in California, that opens up the poem even more. For one thing, it tells us that there might be some autobiographical elements in this poem, since Ginsberg spent a great deal of his life in the Golden State. But it also hints at the poem's origins in the Beat movement, which was based out of San Francisco, where we're betting this little ditty was penned.