Any type of activity that raises the heart rate is considered to be a good form of exercise. Now, with that said, most people do not take up drums specifically for the exercise. This is because there are so many different types of activities that are much better ways to get exercise.
This does not mean that playing the drums is not a good kind of exercise. Quite the contrary, it is actually a fairly decent way to get one's healthy exercise. A typical hour-long jam session can easily burn 180-200 calories.