I agree with the first part of your comment but when you mentioned Jessica and her ice princess image, I have to say I disagree with it. 
I remember that after the debut, SM gave each of the girls a role and an image to present - so Jessica was supposed to be the cold, princessy one, Sunny was the cute one, Yoona elegant etc. But in reality, I remember that after some years the girls would say theose were not true at all. I remember lots of comments that in reality Jessica is very warm and friendly, Yoona turned out to be a prankster, very playful and a bit tomboyish. Sunny got super tired of having to do aegyo all the time.
So while I think that the positions in the group for them (vocalist and dancer) were all good, I personally think that the personas that SM gave them weren't exactly right. I was actually happy that they started to lose them after a few years and show more of their true personalities.
I might be wrong though, cause people change and perhaps there were parts of them that were true to those "personas". Like with Seohyun - she really seemed very proper and a little uptight, and it seems she just grew out of it later on.
Edit: Of course all of it is just my assumptions and observations, because I don't know their real personalities. But they are one of the groups I follow who I feel had to act a certain way in front of the cameras, and that they had certain personas chosen by the company at first.