Most of the poem is set in the generic world of fairy tales. "Cinderella" was originally written in Italian (although some say the myth originated in ancient Greece), so maybe it takes place in a kind of alternate Italy. It's a land that's still firmly in the grip of a monarchy (as evidenced by the prince), which has very strict class rules—as in, the ruling class rules and the have-nots are, well, forced to sleep in ashes.
It's this strict class-based society that really allows the entire story to happen. In fact, it might give you pause when you think about how Cinderella stories still hold their appeal today. In a world where the wealthy have everything, it sometimes seems like money would solve all of your problems. It certainly did for Cinderella: just a few fancy dresses (which would have cost a pretty penny if she'd had to buy them) and she marries the prince!
Of course, how different is this fairy-tale setting from our own? The other setting is, of course, the "real" world—our world. The whole crux of this poem really involves our speaker, who is firmly living in the cynical reality of the "real" world, showing us the dark side of the fairy tale world. We, the readers, are (for the most part anyway) also in the real world, so we know how harsh and disappointing that world can be.
And that, folks, is where these fairy tales come in. They allow us to escape the daily grind (like "diapers or dust") and imagine an easy way out to happiness. Our speaker, though, is all: "No soup for you!" She's pointing out that, even the happy escape of the "Cinderella" story is actually a whole lot less happy than we might have guessed. Bummer.
Still, that might be a helpful realization. Think about it. Do you know anyone who is waiting to hit the lottery, or meet that perfect prince, in order to change their lives for the better? Isn't that a little like hoping for a magical dove to lay a wish-egg on you? Wouldn't hard work and dedication not be better, more practical routes to happiness?
In the end, this poem is all about pulling the curtain back on its fairy tale setting, and exposing it as a kind of lie that people have decided to believe as a form of escape. It may not be the happiest lesson you can learn, but we think that the speaker is really just trying to get us readers to "wise up" and so better deal with our own setting: the real world.