Are you planning to bring a box spring, too?

Long story short, a comforter made for interior use is not optimal for camping. A sleeping bag will be lighter; the down in a sleeping bag is compartmentalized to improve efficiency; the zipper will have some extra protection to keep the warmth in; the shell is water-resistant, so it keeps the down drier (and therefore more effective and lighter); the shell is also made to be compressed, whereas a comforter is not; and the zipper will allow you to regulate the temperature.

Although you could probably get by with a comforter, a rectangular sleeping bag will be superior and not much more expensive.