Can a bad valve cover gasket cause smoke from the exhaust muffler

Roro Valdez, no it cannot. There may be some oil dripping onto the exhaust (header) and most of the smoke would be in the engine compartment. Smoke coming from the back of the exhaust pipe is caused by the combustion itself. It could be burning oil or coolant. You need to take a look at your spark plugs and see what those look like at the electrodes. If they are blackened, oily and sooted your engine is most likely burning oil. If they are kind of rusted looking it is most likely coolant. The repair of this will be depended on what you find out.