In actuality, there is no real definition of what makes a dom or a sub ‘good’ or ‘bad’. It’s not as easy as being awarded a gold star for getting an A on a test or being sent to the naughty corner after continuing to chat after being reprimanded not to (although a naughty corner may or may not be appropriate in this context).
What makes a dom or a sub ‘good’ really does depend on the dynamic that two people share, among other things. (this relationship is often referred to as d/s (dom/sub.)