I think I am confused. The terms "approach shot" and "moonball" with "heavy duty topspin" are a bit confusing when used together.
Are you simply talking about getting a ball that you can come in on and hitting a looping topspin shot deep to his backhand?
Anyway, normally, you want to keep the ball low as you approach. If your method was popular then a lot of people would be doing it. However, if it worked for this opponent then it worked! You found a way to beat him. Sounds like he made a lot of errors trying to deal with that ball.
But I wouldnt count on that strategy working all the time - especially with better players. Also, because not all the time your going to keep it deep and your margin of error increases as you have to swing harder to generate that kind of spin with a shorter court!
The game of tennis is about percentages. How much higher chance would the percentage be for someone having to hit up on the ball you gave them on a low shot vs. a high shot? Since your coming to net, what shot do you prefer to volley, something that is rising or something that could be dipping or diving at your feet? Remember some people can take that ball and chip or power slice that passed you or at your feet.
That is not to say that you cant mix it up and if your good at it - keep trying it until you meet someone that can handle that ball.