Yes, 30 minutes of dancing is generally good cardio. However, the most important thing is the intensity of the workout and the duration – aim for 30 minutes and aim for your heart rate (in beats per minute) to reach 70% max capacity periodically during those 30 minutes. In other words, if your max heart rate is 180 bpm (which would include many Olympic athletes), then you want to hit at least 126 bpm to be in a good zone.