It's a good idea, probably less wasteful than heating up an entire room. You could achieve similar results with just a hot water bottle under a blanket in your lap (you are not immune to hot water bottle propaganda).

I don't think I can easily access one because they are not a thing in Europe, so I never tried one, but I did try the hot water bottle thing, and thanks to that I keep my home at 15°C in the winter without trouble, so I halved my gas bill.

They look comfy if you are supple enough to handle sitting on the floor with your legs stretched under the kotatsu I guess. My back and legs would hurt for sure, since I can't sit comfortably on a floor.