I agree with DukeOM here. Kate is going to have to upgrade her appearance dramatically if and when she becomes Will's wife. JMO
And she will. Remember CP Mary before she married? And other CPs as well have found there own style when they married. It takes time. She dresses appropriately to her age and her hair is similar to other women her age. She usually looks quite nice. William must love her, or they wouldn't have spent all these years together. That's all that matters. He is quite a shrewd judge of character from all accounts, and she suits him. Whether they eventually get married is another question.