We've had that come up here from time to time, and honestly I don't remember what the consensus was. Seems to me that in order to make a good decision on that question, you'd have to keep in mind the first goal of Mama Heating Pad....to provide an environment as close to a broody hen as possible. In order to do that, you want the softness of the heating pad against their backs. You want that heat right at their backs, too, not at their feet, and it seems to me that someone here wanted to use the seed sprouting mat on the bottom, not above the chicks. Heating pads are designed to be used against clothing, furniture, etc, and I don't know enough about seedling mats to know if they would do well covered with the fabric of towels or pillowcases, and I don't know how the heating elements are distributed inside them either so it's hard to say how they would do draped inside or outside a metal frame.
I'm hoping someone with more smarts and experience than I have can chime in here, but if it was me, well, I'd stick to a heating pad for chicks and use a seedling mat for seedlings.