Neither, actually - I got the impression that she simply didn't know how else to react because it was all too overwhelming; finding out that you're royalty and that you're betrothed all at once! I didn't really see her reaction as responsible (going along with something life-long that had the potential to make her and her future husband unhappy) nor spineless (it's not like there was anything she could do about it that wouldn't result in utter chaos). I mostly saw it as confusion.