"Did his attorney really argue that his client was too dumb to be a hacker? Since when did the law require a certain amount of expertise before a crime has been committed?"

If you can't actually do the crime, how can you be guilty of it? If you're dumb, how can you be "a criminal mastermind"?

Ability

Opportunity

Motive

Are needed to be convicted.

If he's dumb (wrt hacking/cracking systems open) he doesn't have the ability to BE a hacker/cracker of a system to open it.

Now if they want to do him for computer intrusion but NOT under the current media meaning of hacker doing so, they can and then "he's too dumb to be a hacker" isn't a defense because you aren't claiming him to be a hacker.