Why was King George such a bad ruler?

You didn't specify which George, so I'll assume you mean George I of England.

He wasn't a bad ruler. He was unpopular during his reign, because he wasn't a native of England (he was the nearest living Protestant relative of Queen Anne; she had closer relations, but they were all Catholic and barred from the throne by the laws of succession). He spoke fluent German and French, but his English was reportedly not terribly good particularly in the early part of his reign, and possibly as a result of this he appeared somewhat "wooden" in public. He also doesn't seem to have been entirely comfortable in the public spotlight.

However, since the majority of his cabinet was fluent in French also, he had no real difficulty in communicating with them and there's no evidence that he failed to understand what was going on. He was a progressive ruler who allowed fairly free (by the standards of the time) criticism of himself and his reign.

If, by chance, you meant George III, the George the American colonies rebelled against, he was also a pretty good ruler who got a "bad rap". He did eventually become clinically insane, and the end of his reign was colored by it, but the American Revolution occurred well before the worst of this.