Wrong. I mean, it can be if we’re talking about super thick, bulky lambswool or tweed, but even then if it’s unlined, it should breathe and regulate temperature better than equally thick cotton (or, heaven forbid, cheap synthetics). Wool is nature’s high-tech fiber. It wicks, it reduces friction, and it breathes. A loosely woven, lightweight, tropical wool suit jacket or sportcoat will be much, MUCH more comfortable than cotton chino in the heat. Same goes for socks in the summer. Skip cotton.