I am a stepmother, and in trying to think of things from her side (the stepmom's), perhaps she is just trying to bond with your child, which is a good thing since she will be a part of her life. I used to do my stepdaughters hair, braids and buns and such, and paint her nails. It was to give me time that was just with her and girly. I in no way intended this to make her grow up too fast or to sexualize her from a young age (things I am much against). I know what you mean about the make up and the tatoos and the sexualized culture today, but unfortunately you are not in full control, and you are going to have to give a little too. We had my step daughter half the time, and she would come to us with the blue toenail and hair and tatoos from her mom's. It would drive us crazy, but in these situations, where a child is not in one home, there is no way that everyone is going to like everything that the child is exposed too.
This is the conclusion I came to for myself with these things.....tatoos are meaningless to children this age (I just put one on my son's hand while writing this). I can deal with tatoos on the hand, but not on the middle of the back (tramp stamp style) or the shoulders, etc. I think if my son like elmo and gets an elmo tatoo on his hand it is no different than a sticker, or a stamp he might get at school. If he is putting it on his arm, or calf to mimic adults with tatoos, that is another story and I won't put tatoos on them like that. Nailpolish is enjoyed by most girls. If she is putting black, or red on her I would object, but if it is pink, or light purple, I am not sure it would bother me personally. You can always tell your daughter it is chipping and needs to come off. Or, tell her nailpolish is not allowed at your home.
It is easy to get worked up about every little misgiving. People have different values. Someday it is going to be can the boyfriend go in her room -- yes at dad's, no at mom's. Or will dad or mom put her on birth control. Those are the serious issues (that we are facing now). It makes nailpolish seem very minor.
My advice would be to try your best to get along with the stepmom. You cannot "let her" or not let her do things. You are not her mother and you don't have control over her. You unfortunatley have to tolerate her and her influence on your daughter (as long as it is not harmful to her).
I get along great with my husband's ex. We are polar opposites, but have the commonality that we both love her daughter. I cannot let her morals/values/lack there of, etc. stand in the way of what is most important, and that is the well being and happiness of the child.
Good luck, these are such emotional issues.
B.