For me, music is second only to math, so I suppose I'm a bit biased. However, using the digits of pi is not really math. It is more like mysticism. There is nothing special about that constant. I mean, you can look at the digits with a different base and it is then completely different, and since there is nothing special about base 10, there is really nothing special about those digits. I think writing this way, which isn't scientific in the least, shouldn't really be considered to have anything to do with math.
It is amazing how much creativity and art is involved in real math. Writing music in a mathematical way is great, and for me is by far the best way to express my self (although I am fairly new to composing, so this may change, but I doubt it). But simply using an algorithm to write and doing nothing else doesn't involve much, if any, mathematical thinking.