Cynicism is Anne Sexton's hallmark. Indeed, it's the hallmark of most of the so-called "Confessional" poets (Sylvia Plath for instance). If you've already read the "Line by Line" and "Sound Check" sections, you'll have a pretty good idea what we're talking about.
Sexton's style is sarcastic and curt. Phrases like "that story" (5, 10, 21, 109), "as you all know" (41), "that's the way with stepmothers" (55), "rather a large package for a simple bird" (62), "blood told as blood will" (89), and "darling smiles pasted on for eternity" (107) convey a kind of disdain for the overly pretty, sentimental world of fairy tales. It's like Sexton is saying, sort of under her breath, "That's not how the world works. The world is way uglier than that. Deal with it." And we have to.
Sexton certainly had to. This hallmark style of hers was the result of a life plagued by abuse, alcoholism, and manic depression. She didn't have it easy at any point, and it's obvious that she's bitter about people swooning over a princess who got everything by merely enduring a few chores. The value of her poetry, though, and this poem in particular, is that this cynicism is an effective tool to wake readers up to the reality of their surroundings.