Literary devices range from the simple to the complex, and Marge Piercy's poem "Barbie Doll" has several of them. A simple one can be found in the third stanza:
Her good nature wore out
like a fan belt.
This comparison uses like or as, so it is a simile.
A more complex but also more significant literary device is the use of symbolism. The title is "Barbie Doll" and it is this plastic, overly perfect object which is a symbol for everything society expects of a young girl (and older women as well). It is an unrealistic standard and an unachievable goal, which becomes obvious when the young girl in the poem cannot live to the standard and takes her own life. The symbol of the "casket displayed on satin" and the girl in her "pink and white nightie" is clearly representative of a real Barbie Doll in her pink plastic packaging.
Finally, the imagery (another literary device) of the poem is that which appeals to the senses. The cutting, for example, is a visual, tactile (touch), and olfactory (smell) image. There are plenty of others to be found, including the visual image of "fat nose and thick legs."
While it is true that this is an unconventional poem in that it does not have the traditional poetic devices of rhyme and meter, it clearly utilizes other literary devices effectively.
The poem also makes use of a literary device called synecdoche, the substitution of a part for the whole. Despite the fact that this woman is intelligent and strong and capable and healthy, all anyone around her can see is "a fat nose on thick legs" (line 11). Obviously, when she walks into a room, people don't just see a big nose on top of big legs: they see a whole person. But because they focus so much on her nose and legs, despite all of her other valuable and important qualities, she feels as though this is all she is. Her two parts, nose and legs, replace her whole self in this line in order to emphasize that this is how society makes her feel: that only her physical attributes are important, and that hers are radically flawed.
In the end, the poem's irony is what makes us feel like we've been punched in the gut.
In the casket displayed on satin she lay
with the undertaker's cosmetics painted on,
a turned-up putty nose,
dressed in a pink and white nightie.
Doesn't she look pretty? everyone said.
Consummation at last.
To every woman a happy ending. (19-25)
We don't know exactly how the woman has died; perhaps it was when she went under the knife to have her nose changed from "fat" to "turned up" and when she "offered [...] up" her thick legs. At any rate, she has succeeded, finally, in gaining society's approval, though it sounds as though seeking their approval is actually what killed her. It is extremely ironic that her death is referred to as "a happy ending" because most of us would not consider the death of a young woman to be a happy event; however, it is happy because, though she is dead, she is finally acceptable to society, and they find her beautiful. Since beauty was always the most important thing, the woman has reached "Consummation at last," the ultimate end.