It's not something I would really stress about. As the others have mentioned, the last thing you want to do is create a body image issue that sticks around for life! A bum is a bum, we all have one. I agree somewhat with a sanitary issue, in that her bottom may be touching a surface once in a while. It's not something I would have thought about though. And I don't know how much of an issue it would really create, germ-wise.
That said, I would speak to her about sitting so that her undies don't show, etc. I mean, as a girl, you have to learn eventually that it just doesn't fly to show your goods to the world! I think she is old enough to know/learn this.
If she is doing things you feel inappropriate... that is another issue. If she is showing kids her private areas or her bare bum, or doing otherwise age inappropriate things with the dollies; THAT I would cease immediately.