As you can see from the preceding responses, whether or not cotton is appropriate is all about weather. In hot desert conditions, cotton is fine and probably the best thing you could wear. In cold, wet weather, it is worse than useless and I mean that literally-it has no insulating power, sucks the heat from your body and stays wet.

In extreme cold, as Echterling noted, cotton is used for outerwear, but we are talking sub freezing weather and usually sub zero. Many deep winter campers prefer it to synthetics because it breathes really well and is much more flame retardant. That is important around campfires and stoves, which deep winter campers often use. It is also used for winter tents as well such as those made by Snowtrekker.

Having said that, none of my winter gear is cotton, including underwear, because I'm not out in super cold weather and not using a campfire.

So, the answer to your question is "it depends."