If this young lady is totally wonderful is so many ways, then I have to assume the tznius issue may be a matter of community standards.
Additionally, her admirable qualities lend me to believe that she may not even be aware she is violating tznius. After all, I am sure she is not dating you in isolation of her fine family. They must know how she is dressing.
Perhaps the intermediary/shadchen can handle this point in a professional and non-offensive manner. I am sure it is a very common concern.
On the other hand, since I doubt she would flagrantly violate an acceptable level of tznius, maybe you possibly might have to adapt.
Her wanting to look appealing to you is not necessarily a fault on her part.
Rabbi Miller, zatzal, spoke about when the mon fell from shamayim in the midbar, something else came down too. It was makeup; enough make up to cover one eye (because that is all that they revealed).
Rabbi Miller went on to say it is very important for a wife to look attractive to her husband. A shidduch date may very well have elements that shadow that sentiment. She may have even been told to look more appealing than usual on her dates with you.
As I am sure you know, there were other times in our history where women were obliged to make the effort to look extra attractive; more than ordinary for their husbands because life was difficult. Again, a shidduch date may be an acceptable place to begin to display a small degree of that kind of sentiment.
I dont know the full situation, but it’s important to work with someone to try to get a handle on this very important standard as you proceed.
Hatzlacha!