You kind of have a point. First of all, it's not to detract from Kara's ability, she is one of the best dancers Strictly has ever had. But, had she been anything but apologetic, humble, demure and un-threatening, I doubt she would have been so popular; Kara only came into her own during the last month or so of Strictly where she truly excelled herself (before that she was rather wooden) - if she had been consistently that good, I doubt she would have been as popular, and would have been deemed a 'judges favourite', which would pretty much assure that they wouldn't win.
For unfathomable reasons to me, the mass British public especially, do not like people who 'own' their talent, and the more talented they are, the more humble they need to be about it to be accepted – otherwise you'll find a large section of society who feel the need to 'bring them down a peg or two', like it's some sort of sport. If you happen to be a talented woman, this seems to be even more so. The funny thing is, this kind of attitude towards women, seems to come from other women more so than men, or at least, they are more vocal about it.
The fact that she was edited as 'ditzy' certainly helped her. Either way though, she was an excellent dancer.