Frankly Nina is smart, and her maturity is because of her mental age, that of her soul's. But she is so reluctant to trust because her position in the novel is one that could easily get her killed, IF she makes the wrong moves, even with the king's favor, which is beneficial in some ways, but EXTREMELY scary for our FL who doesn't know how things will turn out from where she is now since she has no intention of sacrificing herself for the priestess. So all in all, our FL is a damn queen even though she's literally in a child's body.