What has she finally achieved in death that she couldn't in life in "Barbie Doll"?

Marge Piercy's poem "Barbie Doll" is the tragic account of
a young woman who grew insecure and dissatisfied enough about her appearance to actually
take her own life. It begins with a few careless comments about the size of her nose and
her legs.

Then
in the magic of puberty, a classmate said:
You have a great big nose and fat
legs. 
 

What we
understand as outsiders looking in is that she is actually "normal" in every way; but
kids are cruel and thoughtless and these two body parts become a source of misery for
her for the rest of her short life.

The answer to your
question can be found in the last stanza of the poem. She is described for us as
follows:

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.

She is pretty in
pink and her nose does is not big, as a classmate said to her years before. She is
beautiful in her death. The next line is the key.

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Doesn't she look pretty? everyone
said.

What she finally
achieves is approval for how she looks. The kind words and compliments come too late, of
course.

Consummation at
last.