I can't imagine you could find an adult -sized saddle to fit an animal that small. If the saddle is too big, as I am sure you have seen, it will rub on the mule's hips when someone get into the saddle. Bareback is probably your only choice.
I know I would never consider putting an adult on a mini horse that small, but donkeys being so much stronger, the miule might be able to handle it for short periods???