The title of this poem is pretty straightforward—"Cinderella," just like the fairy tale. In fact, almost every single poem in Transformations has the same name as an actual myth or fairy tale.
So that's pretty simple—but if you think about it, it does something kind of strange to the poem. If you give a poem (or short story or novel) the exact same title as a famous story, you set up a particular expectation in your audience. They will expect, naturally, a story that looks a lot like the one they're familiar with. "Cinderella" certainly does this—we think we know exactly what we're getting.
But, as we found out in the "Line by Line" section, we have no clue what we're getting, not really. So by titling the poem so simply, Sexton is setting us up for a whole new re-imagining of what we thought was a familiar story.