Teaching is something people go to university for. Its not just something you can decide to do overnight. Personally I think C & T people in at jet level should have at least a certificate IV (tafe) in workplace training and assesment. Of course most employers wouldn't dream of this sort "expense" which is actually an investment.
In 15 years of flying I've come across 2 people that were absolutely brilliant instructors. They didn't have any formal qualification from memory but were just outstanding professionals of the highest order.
So to answer your question: Are all "good" pilots good instructors? My answer is NO but they certainly have the potential given the right training.