We see him first in the title and he's not what we normally expect when we imagine the archetype of an emperor. He's really more of an idea that's getting at all of the phony associations we have when we imagine emperors. This guy isn't in charge of an empire; he's the boss of… ice cream?! We don't know how you get such a position, but we imagine that it helps to have a sense of humor—particularly about yourself. Since he's also the star of the title, we begin the poem with a playful sense and a hint about what we should really consider important: enjoying life and worrying less about things like conventional emperors and other such matters that are based in appearance.
Title: This dude is the star of the show. Whatever the poem offers us, we read it with the image of this emperor in the back of our heads (at least we should, anyway).
Line 8: Not only is he an emperor, but he is also "the only emperor," which gives us reason to suspect that our speaker is making a pretty strong statement here about this fellow's power and importance.
Line 16: Finally, we see him for the very last time in the poem within the last line. The same phrase is repeated again: "the only emperor is the emperor of ice-cream." Notice too that the word "emperor" is repeated twice each time we see the phrase. Again, this refrain suggests that Stevens is driving the point home with the absurd image of the "only emperor" being an "emperor of ice-cream." What we end up with is the idea that there are no "real" emperors. They're just a bunch of guys with clever-looking costumes who do a good job of convincing us of their importance. The emperor in this poem leads us to consider how, if we're constantly hung up on appearances and playing roles, then we fail to appreciate what life should really be about: living!