Mr. Satan was a professional martial artist. Surely, he was also a buffoon, but from what we've seen he is definitely very strong and skilled by the standards of someone who was never trained by someone like Kami or Kaioshin. In Dragon Ball series, he would probably be a decent opponent for Goku or Tien.
He also wasn't completely stupid - you don't become champion, millionaire and world star by not knowing what are doing.
In the real world, it takes a mixture of raw talent, hard work, and passion to achieve such level, and we have no reason to assume that it's any different in Dragon Ball universe.
Now, to finally get to the point:
- How come that he never came across stories about people like Master Roshi?
- How come he didn't watch 21st, 22nd and 23rd Tournaments, where flying, Kamehamehas, disappearing and other strange techniques were common?
- How come he had no idea about Tien (who won 22nd Tournament)?
Satan must have been a young boy or teenager, and if he had the passion for martial arts, people like Tournament Winners should be like Gods to him. And yet, after these Tournaments, Mr. Satan doesn't recognize Goku, Piccolo or the other Z-Fighters.
Mind that I'm not questioning why he kept on insisting that everything that happened during Cell Games was a "trick" - he wouldn't want to admit that he was outclassed - but why was he actually and genuinely surprised.