He's called Eggman because he is egg shaped, some shows actually say that he's obsessed with eggs too. Robotnik has also gone as just plain Robotnik, Dr. Robotnik, Dr. Kintobar, and Sheriff Robotnik. Also in Japan, Robotnik is more or less a wussy.
He's a giggling, ninny of a moron who likes to take over the world and just caters to his "eggy ego" most of the time. The Sonic cartoons in America. actually made him more eviller then he was in japan.
Sonic X shows the same king of doofus Eggman villain as the games show. But the American Sonic cartoons really make Robotnik a good baddie, especially in the SATam Sonic TV show from ABC.
That Robotnik was freaking satanic!