Oh, I understand those elements of her character entirely and I think those are vital in casting her actress. Thorne, to me, just doesn't seem to have the acting chops to pull that kind of character off. In fact, she's one of the bigger examples I've seen of someone casting her due to her having a "pretty face."
Personally? I think someone like Liana Liberato would really work in the role. She's got impressive acting chops and has shown some considerable amount of range in her roles (i.e. serious, intense drama in "Trust," fun romance in "The Best of Me," etc). The fact that she's attractive and looks like a good match alongside Tom is just the cherry on the top.
Heck, watch her mess around with Vine celebrity Jerome Jarre in this video. She's hilarious and actually kinda reminds me a bit of Tom in personality too.
https://www.youtube.com/watch?v=wcD1Ajp8GC0