It is getting worse because she is getting a reaction from you. This is such a common thing amongst kids. The way I nipped it in the bud and made it so my kids did not want to ever do this, explain what boggers are! Explain what germs are inside her nose, strains of mucus carry a multitude of germs that are needing to work out of the body, why mucus happens and that she can get VERY sick if she continues.
Explaining these things helped my kids absolutely be kind of freaked them out to the point they are vigilant in using hand sanitizer or even washing hands if they itch, blow their nose or put their fingers around their nose (yes extreme I guess). I also explained would they want to play with someone that had their fingers in their nose or share toys with someone that picked their nose???
If you give her attention for it, it will worsen. Let her know she cannot play with other people's toys if she is going to do this. Try and keep her hands preoccupied in the times she is more apt to do it. I had the same problem with biting fingernails for a while. When I just decided to explain they will get sore fingers, that they are getting germs in their mouths and eating fingernails was not good for them then let it go, it stopped all on it's own. It was hard not to say anything but ignoring it helped a lot more then punishment or saying something.
Another thing you can do is tell her that every time you see her do it she needs to immediately wash her hands, kind of say "well if you are set on getting all into your nose then you are not going to get everyone else sick, so wash your hands EVERY SINGLE time you do it" and walk her in the bathroom without saying a word when you see her do it.
Good luck, another thing kids do.......! :)