I don't think you can really control how someone else feels about something. You can complement her, you can ask her to set her feelings aside, you can control your input to her emotional system, but you can't control that her mom or an ex or an ad or whoever said something negative about it.
This might be something where you can negotiate. In certain places or whatever maybe she'll agree to go with bare legs (maybe not!) and then in general she will cover them. But there isn't really anything you can do to control her emotions—even in a positive way.