Whether it was the artist or the author who came up with Finne's design, that person has seriously good design sense. The only way to make her look better than she does would be glasses, and so it came to be.
That said, I bet the people with a problem with Finne would be, if she were flat and shorthaired, far more forgiving of her naïveté. (Bonus points if said hypothetical shorthaired flatchest is a bratty, childlike girl--plenty love that type, even when they do nothing but impede/harm the protagonist.)