Terrible casting of Fi

Why did they completely change the character of Fi ? In the books and film she's really pretty, model like, girly etc but still strong and a sense of confidence about her. Was honestly so disappointed with the tv depiction , no offence to the actress but she's just not suited to this role , out of all the characters she looks the plainest and acts like this shy nerdy character, it's just bizarre am I the only one really put out off by this ? What were they thinking...