When your mother told you not to pick your pimples because it would cause scars, she was wrong. It is very difficult to pick a pimple so deeply so that you would get a hole or a raised scar. When you do pick your pimples is causes something much worst, it causes a flat brown stain or discoloration that is very hard to get rid of and also takes a very long time. It may not happen every time you pick a pimple it does happen very often. Usually this flat brown stain is about the size of the pimple and sometimes even larger.
The reason you get these brown stains is because when you pick your pimples the skin is injured and anytime the skin is injured it turns dark to help protect it self. If you do get these brown spots it takes months to get rid of them and requires treatment with bleaching chemicals, creams and even exfoliates like gentle chemical exfoliates like glycolic or even microdermabrasion.
So when your mother told you not to pick your pimples she was right.