Some cats naturally grow very thick fur in the winter, some to the extent that they look like a different animal! However, some don't. My cat doesn't seem to change all year round, but then he doesn't go out much anyway. I wouldn't worry about your cat being cold in winter - their fur coats usually suffice, and anyway, cats seem to be experts at finding warm places!
Their coat usually suffices for INDOOR cats, yes. But a lot of people don't realize that cats can, and do, freeze outside in the winter. My advice is to keep your cat in at all times, especially during the winter. Their coat IS NOT warm enough to keep them up at normal temperature in the frigid winter conditions.