in the early seasons, the creators of the show did an alright job at making a character thats just an annoying, whiny little brat. but yeah, a large portion of americans like whiny brats and are that themselves, so they saw an element of themselves in eric cartman and his "screw you guys, i'm going home" catchphrase and overall attitude is basically the ideology of apathetic, self-centered americans. so i think maybe the creators did too good of a job making cartman an utterly horrible person, since so many chuds began self-identifying with him.