a person can be too tznius but this is not being too tznius this is just what is comfortable for her. sometimes as a female i wear something and look in the mirror and realize i shouldnt be wearing it when others tell me its perfectly fine and tznius. being too tznius is when u see little girls walking around with hair coverings and a million layers of clothing so u see nothing of her it is very sad seeing this (i saw it for the first time this year while in seminary) let your wife wear what is comfortable for her
tznius wise for nothing else but shalom bayis on its own