Marge Piercy's poem "Barbie Doll" is an ironic commentary on the value society places on outward beauty and perfection as well as the potentially deadly consequences of this emphasis. It all starts, of course, with the title. The Barbie doll has long been the iconic picture of what a woman should be. In reality, she is outrageously proportioned; however, the term "Barbie doll" has come to represent the outward perfection of a female.
In this poem, the first stanza introduces us to a young girl, "normal" in every way:
This girlchild was born as usual
and presented dolls that did pee-pee
and miniature GE stoves and irons
and wee lipsticks the color of cherry candy.
Then, a careless but hurtful comment is made in her teenage years, one which will have a drastic impact, however unintended.
You have a great big nose and fat legs.
No matter how talented or able she is in any of the ways that count, all this young girls sees or hears is that she has a huge nose and thunder thighs. Then, one day,
...she cut off her nose and her legs
and offered them up.
By the last stanza we see her all made up and wearing a frilly, feminine nightie. As people come to pay tribute, they commented on how pretty she looked. Now they say it to her, when it no longer matters. Then come the last lines, including the one you asked about:
Consummation at last.
To every woman a happy ending.
This young girl's ending was anything but happy, and this ironic line highlights that fact. It is a facetious (sarcastic) wish that all women's lives would go as well and end as happily as this "Barbie doll's." In fact, it is a warning and reminder that what other people say need not define who you (boys and girls, men and women) are. This Barbie doll was pretty and talented but bought into the lie of a cutting remark; the speaker of this poem warns us not to succumb to such pettiness.