The title sets up a joke: a diet is about what you refuse to eat, yet this speaker's menu is night, a dream swan, and methamphetamine. The poem has only three short lines, and each one adds a stranger dish. "The night is our supper" makes darkness something to be consumed, and the word "our" suggests the speaker isn't alone, or is speaking for a whole group of people who live at night.
Then comes the "dream swan," a delicate, beautiful image that feels almost out of place. It could be a hallucination, a fantasy, or a hope. The last line drops the tone to something casual and familiar: "my old acquaintance meth." Calling the drug an acquaintance, not a friend, sounds wry and a little weary, as if the speaker knows it isn't good for them but keeps seeing it anyway.
The mood is dreamy and bleak at once, with dark humor running underneath. Nothing is explained, and that leaves the reader sensing both beauty and hunger.