Depends on how you apply the mop. If you wet the mop and dab it on you'll not disturb much of the rub. If you actually use a mopping action then yes, you will disturb your rub.
Frankly, I've mopped my ribs once since buying my SmokinTex. It was more trouble than it was worth. Done correctly, the ribs are going to come out moist anyway. If you are looking to add a different flavor to your ribs with a mop, go for it, but do plan on a longer smoke.
My $0.02