If you let them know you weren't comfortable with this and she keeps doing it, maybe you should get the advice of your divorce lawyer. That could possibly be something you could use in court somehow. (?) I think it's immature not to respect the mother's (or father's) wishes on something like...
Some people do this with their own kids and I think that's okay if it's what they are comfortable with. I have seen lots of parents kiss their own kids on the lips, but I never did. I just didn't feel comfortable with that. On the cheeks, the forehead... that's all okay, but not on the lips...