The poem is only clarified in its allusion upon completion of the reading of the second stanza which identifies a "cold" and "dumb" body as common references to a dead body. They're going to have boats on the Lake of Shining Waters—and ice cream, as I told you.
A remnant of her creativity, in the form of an embroidered sheet, now poignantly serves only to cover her face; it, too, is incomplete, as it is too short to fully cover her. Worrying about appearances, or indulging your sweet tooth?
It's a line that's caused all sorts of misunderstandings in the past and perhaps still can't be nailed down absolutely, which is just what Wallace Stevens would have wanted.
The imagery is strong and unusual - an emperor of ice-cream? What language, images, or meanings do they expect to confront? In Act 4, when other characters are looking for a dead body, Hamlet says to Claudius, Your worm is your only emperor for diet.