I thought the meathead thing was just an act.
No acting skill whatsoever was required for his part. He simply had to go on camera and be his usual obnoxious, ignorant, clueless self.
Does anybody remember any other memorable acting role that Rob Reiner played? I remember seeing him in "Wolf of Wall Street" playing Jordan Belfort's dad. It sucked pretty badly, the other characters in the scene were clearly trying not to break character laughing at him and his wooden performance (trying to get worked up over expenses).