Dave Bonta's short poem is hard to pin down, and that seems to be part of the point. It opens with a strange comparison: giving our "outer self" the way the grave gives, which suggests handing something over completely and without answering back. The outer self is the face we show the world, and the poem treats it as something we surrender or leave behind, like a shell.
Then comes the odd, almost comic image of "a thin king / of the afternoon." The phrase suggests someone who rules over a small, fading stretch of time, with a kind of faint, hollow authority. The last lines turn personal: the speaker calls the whole thing a "mission" in which "I do not believe." That reads as doubt, even quiet weariness, about the roles we are expected to play.
The mood is dry, detached and a little melancholy. Nothing is shouted. The speaker simply admits, in few words, that the public self feels like a duty he can't fully trust.