It's a phase but can be a drawn out habit that she'll outgrow. I had the same problem with one of my nephews I babysat. I finally got tired of the poo and made him sit on the kitchen floor with his hands out every time he did it. Same thing with boogers on the wall - I made my son stand there and put his nose on it, even if he had to lean over. Being uncomfortable about standing there or being covered in your own poo for 5 to 10 minutes seem to work well. I didn't yell and I kept the lecture very short because of the ages of them. Oh, and I also put a frame around the booger on the wall and made him show Grandma, telling him if you don't want people to see your boogers don't pick your nose and if you do end up with a booger, use a tissue.
For another bad habit one of my son's had, we kept a laundry basket up in his closet. Whenever he did it we put one of his toys in the basket. It was the "can't have it" basket. He could see what was in there - which was the point. And we took his favorite toys or parts of his favorite toys. When he'd ask for a toy back we would remind him of his bad habit. At bedtime, if he didn't do it all day we would tell him that in the morning he can get a toy out, he went to bed happy that he'll get a toy back and started out the morning happy he got what he may have dreamt of all night.