Nah, it would be a good idea, but how do you ground against a big solar flare? Conductors are going to work like antennas, you’d probably be better off burying the thing than hooking it up to most things we call ground, they’re going to start fires

And the only reason for the ground is to drain the induced currents from the cage before they affect the insides… You could layer insulator and foil over and over again if you want better protection

But yeah, tin foil alone would do something to make sure the microwave is solidly covered. It’s not like they’re a good faraday cage, they’re just an available one