In just a few lines, the speaker admits to a quiet kind of decline. The first line sounds almost like a boast: "I am plausible in my art." But "plausible" is a careful word. It doesn't say brilliant or true, only believable, as if the speaker can convincingly pass as an artist while doubting how real the skill underneath is. Then the sharp turn arrives: "but I rust." Rust is what happens to metal left unused or exposed to time, so the speaker seems to feel themselves slowly wearing down.
The second stanza explains part of why. Time passes, and the speaker spends so much of it "full of sleep." That could mean literal sleeping, but it also suggests drowsiness, inactivity, or a mind that isn't fully awake to its own life or work.
The title, "Mutable," means changeable, and it ties the two ideas together. The mood is quiet, honest, and a little rueful, with self-doubt told plainly rather than dramatically.