In the episode, Carrey was funny without annoyingly trying to be the center of attention. I see your point on the fence bit, but everything else from the backing up while hearing a truck back up, trying on that sweater he found on the ground, to the stevia longshot, seemed like things he would have done without any cameras there.

I stopped acting in high school because I was sick of being around theater kids who were "on" all the damn time. Carrey seemed more down to earth and interested in having a meaningful conversation with Seinfeld. He just also has a knack for seeing jokes where others don't, without seeming try-hard.