The speaker of "Barbie Doll" sounds like the narrator of a modern fairytale, so we can expect that the poem will sound the same in a lot of ways. In fact, the poem reads more like a story than a poem without a set form, meter, rhyme, etc. The free verse allows the speaker some freedom to narrate the girlchild's story without distracting us too much with fancy poetic devices.
But, if "Barbie Doll" reads like a modern fairytale, we need to also take note that this isn't your run-of-the-mill "Princess and the Pea" story. The poem points out that, if the girlchild doesn't sound "perfect" to us, it's not because there's something wrong with her; it's because there's something wrong with us. Oh sure this may sound like a light jaunt in fairytale land, but there's certainly nothing magical and light about the speaker's message.
The sounds at work in this poem help to underling that idea. Notice that the speaker uses devices like assonance ("pee-pee," "GE," "wee," "candy") to help create that airy, rhyming, fairytale sound without distracting us too much from the poem's main ideas. Plus the subtle rhyme adds that super-creepy feeling that reminds us of childhood, only we're seeing things with a weather eye for the not-so-fun-and-innocent.
By the time we get to lines 17-18 (cutting off noses and such), we really hear the gruesome but true severity of society's pressure. Instead of cheering for the happy prince who has found his "perfect" princess, we find ourselves considering the princess's (girlchild) side of the story, and there aren't too many happy endings to be found there. In fact, what sounds at first like a happy fairytale in one way ("she went to and fro apologizing") ends up sounding more like a gruesome nightmare, complete with bodily mutilation and crushed spirits (worn out fan belts). Bad times, gang.