If the rotor rubs, it's either the rotor itself or the hub. The hub could have loose bearings or a bent/broken axle. Most commonly, especially with low-end mechanical disc brakes, the rotor becomes out-of-true due to hard braking. It can be trued with your thumbs, but there is a tool that makes things easier.
Now if your RIM has a wobble, that is a separate issue and should also be dealt with.