Our pumpkin-speaker is really the star of the show. From the very beginning we understand that the speaker is the pumpkin with the repetition of words like "I" and "me." So this poem is coming directly from the source, which makes it sound all the more authentic and also pretty unique with all that personification. After all, it's not every day we get to hear pumpkins talk, let alone recite poems to us.
But we may ask ourselves, why choose a speaker that's a pumpkin, besides it being totally unique and cool? Let's consider a few ideas. First, our pumpkin speaks in a very simple and straightforward kind of way. He's not trying to search for the meaning of his existence or anything. He's just kind of saying, here I am, this is what I look like, oh and since it's Halloween, kids get to carve me some "terrible teeth." And when folks are feeling in the mood for some simple fun, it helps to have a speaker who's as simple and fun as it gets: a pumpkin. Not to mention the fact that a jack-o-lantern is the ultimate symbol for Halloween, which makes the whole poem stick with us even more when we imagine that talking pumpkin.
But even though the speaker keeps things plain and simple, there's still some poetic maneuvering going on, especially when it comes to establishing the poem's setting and how the speaker fits in with everything. For example, he could've just said, "I look yellow sometimes and then I kind of get this orange color." But of course he says instead, "I spot the hills with yellow balls in autumn." Notice the difference? One sounds like it shouldn't really be in a poem, while the other sounds mighty poetic but still approachable in terms of meaning. All in all, there's a balance going on here that's equally simple as it is beautiful, which really fits with the fact that a pumpkin is telling us a poem.
So yes, the speaker's tone is informal, but it's not as if he's just rattling off a bunch of random stuff. There's still quite a bit of beauty and cohesiveness to be had in the poem he's sharing. Remember that it's Sandburg who's creating this voice, so the pumpkin can't be entirely devoid of poetry.