According to Enotes:
What sets Postmodernism apart from its predecessor [modernism] is the reaction of its practitioners to the rational, scientific, and historical aspects of the modern age. For postmodernists this took the guise of being self-conscious, experimental, and ironic. The postmodernist is concerned with imprecision and unreliability of language and with epistemology, the study of what knowledge is.
Plath's poetry is intensely "self-conscious, experimental, and ironic." Plath's poetic voice is two-fold: anger and absolution. At its best, it attacks, purges, and confesses all at once. This is the voice of "Daddy," written in 1962, the same year The Bell Jar was completed:
Daddy, I have had to kill you.
You died before I had time---
Marble-heavy, a bag full of God,
Ghastly statue with one grey toe
Big as a Frisco seal
There's a stake in your fat, black heart
And the villagers never liked you.
They are dancing and stamping on you.
They always "knew" it was you.
Daddy, daddy, you bastard, I'm through.
Here, her words are weapons. Plath bombards the reader with imagery and metaphor: her "Daddy" is a shoe, a statue, a Nazi, a teacher, a devil, and a vampire, and she wants to escape from him and kill him and forget him in one fell plunge. She takes no prisoners, wielding a loose cannon control. There is introspection, but it is without self-pity. There is an undercurrent of playfulness in her grim tone, but it lurks around its margins, as if she were writing the poem with a pink pen in one hand and a kitchen knife in the other. This is the voice that comes in nervous fits and nightmares and leaves after only after a bloodletting.
Anne Stevenson lauds the paradoxical complexity of Plath's poetry, saying it "is all of a piece":
Its moments of tenderness work upon the heart as surely as its moments of terror and harsh resentment. And despite her exaggerated tone and the extreme violence of some of her energy, Plath did, courageously, open a door to reality.
Stevenson goes on to praise Plath's "Lady Lazarus" persona "with its agressive assertion of regeneration, rejoicing in so much verbal energy that the justice or injustice of the poet's accusations cease to matter." This may have been Plath's intention with The Bell Jar: to create a novel of such verbal energy that the decisions or indecisions of its narrator cease to matter. But language alone does not a great novel make. Certainly, Plath's first-time fiction does not contain the same punch as her well-rehearsed verse.
Taken together, Plath's poetry reveals a wide range in tone, from the joy of newborn children to rage against patriarchy--all of which is very post-modern.