Art style and how clumsy the proportion is so fitting for horror.
I was honestly thinking that the art style's really grown on me. It's more emotionally forward than it is technically sound - like the one panel of younger Mirei crying on her bed.
It looks almost childish, but in some ways, it feels even more gut-wrenching given the depiction of her spiraling int depression in the wake of her best friend's removal from her life--by her own hand, in a sense.
Horror definitely would be fitting with the look of this, but it almost reads like a memoir of a child's recollection, like how the memories in your head can be a bit fuzzy and distorted; so too is each page of this story, giving that sense of things being "not quite what they seem". Definitely apt for being horror-coded, but even just in a psychological thriller sense, it's very appropriate.