There are assumptions for sure, as to the materials of the sloth, and how it interacts with the microwave energy. Will the eyes heat? Probably. Will they heat before the rest of the plushy erupts into flames? Not sure, but it'a testable, just not in my microwave.

There are many sections of the electromagnetic spectrum, but we are dealing with microwave energy in a microwave. Communication signals bouncing off the ionosphere and RF generated by car ignitions doesn't seem relevent to the discussion.

The RF, as electromagnetic energy, will induce current in metal objects that cut across the path of propagation. Yes, size, shape, material are important, which is why the plushie doesn't immediately catch fire.

If you allow microwave frequencies to cut across a human body with sufficient output power, you will heat that body and cook them with similar effects as food in a microwave.