Good lens on a mediocre camera or a mediocre lens in a good camera?

Today’s lenses and cameras are so good that you can start with mediocre lens on mediocre camera and still get excellent photos. I’d be careful using “mediocre” adjective to both cameras and lenses. Any camera/lens is the best for a certain set of conditions. The so-called mediocre camera/lens combination will likely cover very wide spectrum of conditions to get good photos with the exception of some extreme situations to satisfy most requirements of a hobby photographer.