In contrast to all the finery (expensive stuff) that appears in the poem, there is, of course, dirt—lots of it, and not just in the Cinderella story. The poem is constantly comparing images of dirt with images of lovely clothes and jewels. By making these comparisons, the poem seems to say that to not have money is to necessarily be dirty. Given the cynical nature of Sexton's version of the fairy tale, she seems to think that this viewpoint is ridiculous.
Line 4: The first "example" story is about a plumber, someone who makes a living by dealing with some of the foulest dirt there is. You probably picture him as a dirty guy in overalls. The "toilet" in this line is a very purposefully chosen image of dirty, manual labor and we're about to see a few more.
Line 9: The second "example" story involves a nursemaid, someone who deals with babies and housecleaning all day. Like the plumber, her job involves a lot of dirt—basic household dirt (dirty dishes, dirty floors and bathrooms) and also, as the line indicates, dirty diapers. Yuck.
Line 11: At this point, the image of the milkman—the third profession that involves getting your hands dirty—is starting to give us something that's almost like an allegorical story. All of the example stories are interchangeable, creating a master narrative of "menial worker experiences a stroke of luck."
Lines 17 and 20: Even the sound of the word "charwoman" implies dirt—to char something is to burn it, which creates sooty dirt. And the word "mop" doesn't make us think of a nice, shiny, new mop, does it? Nope. It's an image that most likely conjures up something covered in dirty water.
Lines 30-31: These lines are the first in which Cinderella takes her place as the dirt-covered heroine of the story, sleeping in front of the fireplace and "looking like Al Jolson" (see the "Line by Line" and "Shout Outs" sections for more on him). It's a simile that plays off the images from the beginning of the poem. In Cinderella's case, though, it's temporarily reversed. Her misfortune has, quite literally, turned her dirty.
Lines 64-67: Here the image in the poem showcases the contrast between dirtiness and cleanliness: the stepsisters don't recognize Cinderella when she's clean. It's as if she's a whole other person, which might make sense since cleanliness usually symbolizes wealth, or social status.
Line 103: At the end of the poem, when the prince and Cinderella are living "happily ever after," we have a pronounced lack of dirt: no "diapers" or "dust." These two terms stand in for dirt and grime in general, so this is an example of synecdoche (when a specific thing represents something larger).