Is a plush mattress like this bad for your back?

Not when the support underneath is built for it. This plush pillow top sits at 2.5/10 firmness but scores 7.1/10 on spinal support because 1000 pocketed coils of heavy 13.75-gauge steel push back to keep hips level while the 5-inch plush top cradles shoulders. It suits side sleepers and lighter bodies best; strict back or stomach sleepers, or heavier builds wanting firm pushback, will likely prefer a firmer model.