....Or, more likely I think, did her family and everyone else think she'd be able to grow into the role, knowing that it was likely decades before her husband would become King and her role would become that much more public and full of responsibility? How was anyone to know how enraptured the public would become with her, and how easily she'd take to the more glam side of things and the charity work and thus increase her fame far faster and to a greater degree than anyone might have thought?
Maybe Charles himself thought she would grow into the role and he would grow to love her - it seemed at the time that the priority was a well-bred Englishwoman who would bear him children, not for him to find his perfect love. She was so young and perhaps too starry eyed to really get it, but I can't imagine that she didn't go into it with high hopes for a long and happy marriage either.
Sure, in hindsight it was an ill-fated match, but it's entirely possible that everyone went into it with the best intentions, and only over time did it fall apart, to the blame of no one.
