It is not just a boy thing, My niece is a nose picker. I have been baby sitting her since she was 6 weeks old and she is now 7. I used to find boogies wiped on my wall where she would fall asleep for nap time. I caught her wiping one on the wall and made her clean it off. She also wiped poop on the bathroom wall when she was being potty trained
My sister was horrified because she would also do this at their home too! My 4 boy's never did anything gross like that!!