The speaker is a dead person talking about their own gravestone, and the tone is restless, a little wry. The stone is "too-grave" (heavy, solemn, and a pun on the grave itself), and it seems worn out by its job as a marker, with its cross like a bull's-eye. Stones, the speaker says, are sleepy things that know nothing of flight. Yet this one might topple in any wind, and the speaker watches that possibility with something like hope.
The ship at Sutton Hoo, a buried vessel, gives the image of a grave that is stuck in place, a ship going nowhere under the turf. The final lines reveal the speaker has already escaped: they "traded" their corpse for "fire's fey wings," which suggests cremation or some kind of spiritual release. The title, "Mutiny," fits that spirit of rebellion. The mood is playful but eerie, mixing gravity with a longing for motion and freedom.