unlike others, i actually find this heroine very convincing and genuine
why? she might have memories of her past life, but her brain is still developing like a normal person's brain would, meaning her brain simply isn't developed enough to grant her the insight of a 25 yo she "should" have as she has that amount of life experience
she acts like a child because she is a child, the only difference being she has memories of her life before reincarnation
she is cheeky and brat-ish like a angsty teen would be - because that's the age she originally died at... to me, it makes so much more sense than the super genius babies we encounter in other reincarnator stories