In reality, he was neither mad nor tyrannical -- at least, not by the standards of the era. No British monarch has wielded more authority than their parliament since 1688, but it's a lot easier to burn an effigy of a king than it is to sit down and have a learned debate on the intricacies of parliamentary democracy. The American colonies actually had the same representation that much of England did, like Manchester and Birmingham. And while it was reasonable to object to that, it doesn't mean America was some hellish dystopia ruled by a cruel madman.
Revolutions are complicated, but George's "madness" is touted as the singular reason Britain lost its American colonies. In reality, he was a diligent leader who showed an incredible attention to detail. There is some evidence that he had bipolar disorder, but that doesn't inherently make someone intolerably unstable and unable to lead. George would retire from public and political life whenever he felt ill. And when he was on his game, he was an accomplished agriculturalist, military mind, historian, and astronomer. He did try hard to hang onto the colonies (and what ruler would simply let a big chunk of their territory go without a fight?), but he was also willing to work with America once it became clear that the war was lost. That's a far cry from this image of the king:
The Samuel Goldwyn Company