“Good girl” is one of these phrases that is so common in the lifestyle. It’s used so often that it almost has become without meaning. Like people say “I love you” with the same intonation of “Pass me the butter please”. My daughter said that for years on that tone and I always replied to her “It’s nice weather outside isn’t it?” I wanted to show her that these words have no meaning if you don’t feel it.
It’s the same with “Good Girl”. And when you use it right, and there is one way to do so and that is really mean it, then it’s the biggest compliment you can give as a submissive.