There is often a bit of a difference between our normal, everyday meaning and understanding of "insane" and the legal definition of that term.
In a common, everyday way, it sure seems like anybody who would place such great stock in the fictional shit of comic books and then act it out in the horrific violent, deadly way that Holmes appears to have acted would qualify as "insane."
But that doesn't translate into a winning legal defense, necessarily.
-- Colorado Criminal Law Understanding Basic Colorado Criminal Defenses - Criminal Attorney Specializing in Sex Crimes Law in Denver, ColoradoIn Colorado a person is insane if the person is suffering from a mental disease or defect that makes the person incapable of distinguishing right from wrong.
It is often very hard to "sell" that a guy who supposedly couldn't grasp the distinction between right and wrong was able to plan out his conduct including efforts to conceal or escape.
My guess (pure guess) is that it is going to be damnably difficult for this "joker's" criminal defense lawyers to make out the claim before a judge or a jury. I suspect the effort will fail.
I think you are also misinterpreting. Holmes is not insane because he committed acts of mass murder and was able to plan the crime and a get away. He is insane if he imagines himself to be a cartoon character. If, acting as the Joker, you can prove that the Joker was able to grasp the distinction between right and wrong and was able to conform his conduct to the law, now you have an argument. Had Holmes remained under the delusion that he's the Joker he would never stand trial. He wouldn't be competent to stand trial. Recently Holmes shows indication of no longer being under that delusion. He doesn't know what happened and doesn't believe he's the Joker. Now he is able to communicate with his attorneys and participate in his defense. Now he can be tried.
If this keeps up, this is going to become more like a multiple personality syndrome defense in which one manifestation of personality has absolutely no idea, or memory, of what any of the other personalities have done.