Why did Erik and Arthur look like fools in the final football game?
Erik and Arthur look like fools in the final football game because Brian Baylor, the boy who plays center, kept throwing the ball up way too high or wide, so that Erik and Arthur had to jump and dive to get the ball, looking stupid the whole time. It's clear that Brian did this on purpose, because before this game, he'd demonstrated excellent skills in throwing the ball properly.
We find all this out on pages 231-232, when Paul discusses the newspaper's coverage of the game with his dad on the living room floor, the morning after Thanksgiving Day. The spotlight for the entire game is focused on another boy, anyway, meaning that Erik didn't get any recognition for his performance in the game in addition to looking foolish during it.
Of course, Paul finds the whole situation funny. He's tired of seeing his cruel brother soaking in the glory of football success, but he's trying to hide his amusement from his father, who is irritated that the news article doesn't portray his other son in a better light.