Even if thats true, I bet you more people more about Donald Duck, then Mario and Mickey combined. ...Thats only because Donald Duck was so extremely popular - esp after WW2, when the first cartoons a lot of Europeans saw was Donald Duck cartoons - Plus all the comics that have been published over there.
Haha. You're probably confused to what I am talking about now.
Actually, Plane Crazy--the first to be animated and the third produced--aired first. Steamboat Willie came later and is popular, because it was the first cartoon with official sound (beyond squeaks and stuff) and was aired in theatures for its original airing.