I think Apu is targeted instead of any other character is because aside from family members, Apu's the only regular main Indian character in the show. You could make an argument that Dr. Hibbert is a play off billy cosby's character (he is), but there are other black regular characters not related to him. bumblebee man is latino and can be stereotypical, but dr nick exists as well. Even luigi, who is absolutely an italian stereotype, doesn't have much clout because there are other italian characters (ie. fat tony and his mob)
And come to think of it, aside from Apu the only other asian simpsons regular i can think of that one chinese buisness woman. that's what we should be arguing about.