Four short lines, and the speaker is simply listing what their hat is. But the hat is never described as an object. It isn't wool or leather, red or black. Instead it's angry, weary, troublesome, staid. These are feelings and temperaments, so the hat starts to sound less like clothing and more like a person, or maybe a stand-in for the speaker's own head and mind.
The list also changes direction as it goes. It begins with personal moods, then moves to "a yesterday hat," which suggests something old and stuck in the past. The last line jumps to "a war hat, a Big Brother hat," and suddenly the tone turns political. The hat now seems to carry authority, surveillance, or conflict. The opening "fat hat" is playful, with its bouncing sound, so the poem feels half joke and half unease. The mood is wry and a little oppressive, as if something silly has quietly grown heavy.