My first successful cultured mozza was with powdered milk (+ a little bit of coconut oil). The cow was dry, I needed a cheese, and we had come into the powder and oil for free. I added a bit too much oil, but the mozza turned out well enough. It was a huge confidence boost that I could actually make a mozza. I used linuxboy's mozza recipe, from somewhere on the forum.
The flavor is not as good as with the cow's milk, but I wouldn't characterize it as "flavorless." It didn't have as robust a flavor, but it worked well for pizza and cooking. It was probably my 2-year-old's favorite mozza I've made, but that's mostly because he doesn't like stronger flavors in his cheese yet.
If the milk is free I would (and do) use it. If you're buying milk, go liquid.