I think the dealership messed up my valve cover gasket when they replaced the cam sensor. Now after I complained about high oil usage they found the valve cover leak, could they be at fault?
Anything is possible, but... There is no reason to disturb the valve cover when replacing the CMP on your car, so they probably didn't cause the leak. That is of course assuming that it is in fact the valve cover gasket that's leaking. Improperly installed, the CMP would certainly leak.