Although there really isn’t such a thing as being too tznius, there has to be a line separating tznius from unkemptand shlumpy. Tznius is showing everyone that you are so much more than a body, you are a precious neshama. However you can get people to realize this, do that. For example, if you are in say, Hershey park, and you are in a long sleeved shirt and a skirt that covers your knees people will look at you.but it’s not negative attention in the sense that they are noticing your body. Its just the way you are dressed that is attracting their attention.
However if you are actually embarrassed by the way your wife is dressing then you should definitely talk to her. Maybe you are just not noticing the details on the first robe that she felt made it untznius.
I for one, wish I could be on that level.