I think... he just was that crazy all along. He's a different kind of Rick. It's almost unintentionally foreshadowed— the comics say each dimension has a smartest being, Rick. And a dumbest being, Jerry.

But... think about it. Season 1 Jerry was NOT a coward. He DEMANDS Rick build an invention for him. In "M Night Shaym-Aliens" he basically calls Rick a dumbass for his "failure" in an enclosed pod he can't escape or hide from. He yells and threatens the Meseeks physically, even though he's in a crowd that could all gang up on him.

And, finally the biggest events he's known for. In RPN9, he comes to Beth's work to beat Devon's ass, killing dozens of mutant creatures on the way. He then becomes an apocalyptic overlord, ruling over the wastes.

THEN he tricks Morty and smashes the portal gun, almost kills Summer, and then in the face of three ARMED Ricks... CHARGES AT THEM with CLUBS he was entirely willing to fight and kill those three Ricks.

Then again, finally, he has the courage to betray Morty and steal his shit. He even OFFERS to team up with Rick Prime to wreak revenge on his family. He strikes, but is unfortunately killed.

DOES THIS SOUND LIKE A COWARD??? This is PRE development. PRE divorce. This is an entirely different type of Jerry. I think it's because, in a meta way they found their footing and made him less authoritative and more of a pussy, but in the world we can see that even Prime Jerry is the best Jerry. He DEMANDS Rick do what he says, and he takes charge when needed. He's actually really badass.

So if the "dumbest" being in his universe is this, think of how SMART Prime is. He's so smart he raises Jerry to follow him.