Because he wears it for days at at time (the man hardly showers)lol. So it will get messy and smeared due to sweating and prob. itching. His fingers have his war paint all over them.... :D
I think his constant laughter is a contributer to his makeup smearing. But, makeup या no makeup, smeared या not, bottom line is,The Joker is hot no matter what!! HE IS SO SEXI!!!