Is it safe to stuff some insulation above my electric stove?
Use your exhaust fan.  It removes grease, products of combustion and most of all moisture.  Do you just never use your stove top?  I suppose if all you ever do is microwave and use your oven you don't need to.

You can fill the gap with roxul insulation which is fire proof.

Typically your exhaust fan venting will lead to the exterior wall which has a wall cap.  The wall cap has a damper.  The damper usually seals somewhat against the wall cap to prevent drafts and only fully opens when the fan pushes air out causing the light metal flap to push outward.