In my mind, I am seeing fire.
So there will be an electrical arc from the microwave's magnetron to the metal strip.
The mask has fiberglass weaving in it. This will melt. It is also flammable when melted. The webbing holding the mask to the head will also melt. It too is flammable.
The electrical arc from the magnetron to the metal strip in the mask will ignite the flammable materials.
You will then have a nice smoky, awful smelling, and even a little bit dangerous, fire going on inside the microwave, complete with sparking.
Sneakers, at least the old-fashioned kind, had metal eyelets for the laces. Those too used to produce electrical arcing from the microwave's magnetron to the laces. I think the eyelets are plastic in today's sneakers.
Old-fashioned sneakers had rubber soles. Current ones have some sort of plastic material as soles, and a lot more plastic in the body.
Clifford Stoll, in his 1989 book, The Cuckoo's Egg: Tracking a Spy Through the Maze of Computer Espionage, tried nuking his wet sneakers in a microwave to dry then. The sneakers melted and ignited, producing a nice smoky fire reeking of burned and melting rubber.