The 120/60 is too flat for supermoto:
1. It is not absorbing much in the dirt.
2. the sidewall is too low, and the side carcass is too narrow. This resulting less grip in deep lean angles then the broader side carcass at the same lean angle of the 120/70.
3. Supermoto has most of its weight on the rear wheel, hence can not employ a 120/60 as well as a sport bike to get the required grip level.