From the Super Mario Wiki
Mad Boos are Boos that appear in Super Princess Peach. Mad Boos look just like normal Boos except their facial expression is more evil and sinister looking. Mad Boos' behavior are the exact opposite of that of a regular Boo, Mad Boos do nothing when Princess Peach turns her back to them, but if she faces them, they will attack her. They also appear in a rarer Big Boo form called Mad Big Boo.