Then we shall say: "She wore herself in her hair, in her day, And we could see nothing but her huge flop of petal,
In this next stanza, we get what nearly amounts to a eulogy – that is, the speech that someone makes about the departed at a funeral. Even though the poppy as a plant isn't dead yet, the falling of the petals signifies the end of the plant's life as a flower, and the imminent death of the plant as a whole. (Besides, the poem places quite a bit of importance on those petals, don't you think?)
So what usually happens in eulogies? Typically, a eulogy is nearly a celebration – a speech of praise, a memorial to a life just gone. This is no exception.
But it's subtle in its way – the poem could just go out there and say "and we'll all say that this flower was pretty and pretty awesome!" but instead it says "she wore herself in her hair." We can probably assume that "hair" equates to the flower petals, but that's kind of odd, as the petals have been up until this point everything from a carpet to fire to skirts. That's a lot of imagistic jumping around! What's up with that, do you think?
Petals-as-hair makes some sense here, we think. After all, the petals are at the top of the flower, kind of like hair is at the top of our heads.
So then what does it mean for someone to "wear herself in her hair"?
Think of it this way: we all know someone (or are ourselves someone) who has a kind of trademark-style thing. Maybe you have a friend that always wears big earrings. Or bright pink lipstick. Or has a mohawk. An expression of personality, in other words, that comes out through one specific stylistic choice. You might say, then, that someone can wear themselves on their sleeves – literally! So to say that a poppy "wore herself in her hair," then, is to identify those fiery petals as a trademark. The poem's certainly done a clear job of doing that so far, we think.
In fact, it does such a good job of it that we're ready to agree when it goes on to say that "we could see nothing but her huge flop of petal," – those petals have been the center of attention for pretty much the whole poem now, eclipsing nearly all of the other things that are in the poem (seed pod, bumblebee, fly, August, etc.).