The Mary Tyler Moore Show was a comedy that spun off a drama about Lou Grant. M*A*S*H was a comedy set in a bleak war zone. Besides, it's Hollywood-- they might not even notice the Zombie Apocalypse.
Lou Grant had no serious connection to the parent series--notably no MTM main characters ever appeared on the series, so for what it was worth, Lou Grant was a spin-off in name only. At the moment, we know little about TWD spin off, but we do know it takes place in the same world--a serious world, so it is difficult to imagine the spin off will be anything other than a change in location / political commentary, but no overt humor.