This one is pretty clear. "The Emperor of Ice-Cream" takes place at a wake so we know that Stevens is working with questions of life and death. In the case of this poem, death represents a kind of ultimate reality, one that is inescapable no matter how fast your sports car can go. Don't be bummed, though. The poem encourages us to adopt a fundamentally positive approach to life in light of this undeniable reality: live life focused on the moment at hand. While this is pretty hard to do (if we really did that, we'd never have finished writing this paragraph), it can be a good lesson on perspective. Don't get bogged down in superficial stuff because, in the end, none of that nonsense matters. Instead, be attuned to the moment, the way you are when you eat ice cream. Yum! Sounds like some tasty advice. And it's all thanks to death!
Carpe diem is just a fancy way of saying we need to get up off the couch and stop worrying about what Honey Boo Boo is up to. With this poem, Wallace Stevens would agree.
Easy for you to say, Stevens. He may be "the only emperor," but the idea of the "emperor of ice-cream"—living life for the moment—is not a model that any of us can realistically hope to follow.