Unlike spring-hung Shimanos and Simplexes, the earlier SunTours were designed to operate from a fixed position. However, you still need to be able to rotate the derailleur body clockwise to remove and replace the rear wheel. If you always carry a 6mm hex key, just cinch down the derailleur body and don't worry about it. Otherwise, try putting a very small (in both inner and outer diameter) washer over just the threads of the mounting bolt, which will increase the distance between the frame's derailleur tab and the bolt's shoulder. An appropriately thick washer will permit the derailleur to pivot under reasonable hand force, without flopping or wiggling from side to side.