Her physical body is still that of a child, so it's understandable that even if she has memories, that doesn't actually make her an adult.
Basically her body can't catch up to her mind. But she gets a loooot better later.
It still dosent excuse the fact that shes a pain in the ass. Like sure it provides and explination as to why shes like this but that dont mean nun. She has more memories than any kid could have in that world and shes supposed to be an "adult" but it really just seems that before she died she was still a kid that never matured even though she aged.
We don't really know too much of what she's like before she transmigrated (because the book does not talk about it at length); only that she was a typical sheltered young Japanese woman / bibliophile. But when you're in a child's body, regardless of your rationale or memories, you can't help but be influenced by your physical limitations and hormones. Like...my teenager is smart and intellectual and reads a lot but is often overwhelmed by hormones typical of her age and ends up too high strung and emotional (she understands, but she can't control the way she feels), and is exceptionally annoying to calm down. Also. Just because one is well-read, doesn't mean they actually know what to do with the knowledge. You can be book smart and not street smart, which is exactly what Myne is; and they do bring this up in the novels that she's a plethora of knowledge but a useless applicator. If you don't like her chaos gremlin personality, then it it what it is. However, she does eventually become much more likeable as the series progresses.
The plot is interesting but the MC makes this really hard to read. She is too whiny for a character who has read so many books and is consequently an intellectual. I can understand why she would do that though. I can’t imagine being in an era where you don’t have proper books, no shampoo, no showers, no cars(she has to walk to the forest but can’t). I would get frustrated too, but her personality still shows off as annoying regardless.