The poem is a tiny, strange story about mistaken identity. The speaker says a barber has put on the speaker's clothes, and because of the outfit, other people take him for a cook. Clothes decide who someone seems to be, and the barber slips into a role that isn't his without having to do anything to earn it.
The last line is the oddest: "All ate a great deal of nothing." It reads as a joke with a sting. If the barber is treated as a cook, perhaps the meal is empty, a performance with no real food or substance behind it. The title, "Imposter," makes the unease sharper, though the speaker sounds more dry and detached than angry.
The mood is deadpan and slightly surreal, like a dream where everyone accepts a false situation without question. Underneath the humor is a quiet suggestion that identity is fragile, and that appearances can feed people nothing at all.