I think you should first be VERY VERY sure that your concept of tznius is halachically accurate. There is a difference betwen her wearing a skirt that covers her knees even when sitting, but is on the short side versus wearing a skirt above her knees. There is a difference between her wearing bright colors, rather than black, brown, and beige, versus someone who wears a tight red dressd. Is her way of dressing the issue or her behavior? Those are very different things to consider.
As you have not really expressed what your tznius objection is about, it’s hard to understand the problem. But no matter WHAT it is (and again, be very sure that it really IS a tznius problem, and not just your own perception that is a problem), then in my opinion you should speak to the shadchan who set you up, if any, or to her rebbetzin (or another female with whom she is close and with whom you would feel comfortable), and ask her to speak to her about what’s bothering you. You may find that this other person thinks your issue is not based on any halachic foundation, and will help you to see that, or you may find that she is able to get your point across to the young lady without embarrassing you.