Question: Why did Fridge explode when he got killed by his weakness, cake, but not Martha when she gets killed by her weakness, venom?
Answer: The simple answer? It was just a funnier visual to see Fridge literally explode from eating cake instead of just dying. It's just a very minor inconsistency in the film for comedic purposes.
Question: What does Fridge do now if he's still kicked off the football team by the end of the movie? It seems that only Spencer, Bethany, and Martha had some character growth, but not Fridge.
Answer: Fridge absolutely had character growth. He learned to value others as actual friends instead of using people to make his life easier. He learned to have respect for less popular students. He learned that physical strength can't solve all your problems. He learned the true value of teamwork. He never would have learned any of these lessons had he not played Jumanji.
What about Spencer? What kind of character growth did he have?
Primarily he gains confidence and self-esteem. He learns to be more decisive but also seems to have much more courage than before.
The claim that he wouldn't have learned these lessons if he'd not played Jumanji is unfounded speculation. There's no way of knowing what his life journey would have been otherwise.
Answer: Spencer learned to be more confident in himself.
Answer: Allergic reaction.
She did explode (I've just watched it) just differently.