If we describe a lack of tznius as drawing attention to oneself, then by that definition, any woman brisk walking on Ocean Parkway is behaving immodestly.
I think, to a degree, tznius is a personal sensitivity. I used to ask my mother before going out on dates if I looked tzniusdik (and often felt I didn’t because of the makeup I wore, etc.). Still, it would probably be inappropriate to go on a date SANS makeup.
Certainly, a man should not have to look at his wife and hate her mode of dress. You need to bring your feelings to your wife’s attention, and ask her if she could, perhaps, alter the robe, slightly, to meet both of your standards. When in doubt, you could always ask her to check with her rebbetzin and/or friends, who would check both the appearance and the tznius factor.