Yes, it is bad because it interferes with the cooling system by blocking the air vents on the bottom of the PC. If you do place it on a bed or other soft surface, put it on something solid first like a book or a cooling mat designed for notebooks.
Yes, it is not advisable to do that. This is because if you leave it on a carpet someone might step on it or the dust in the carpet might affect its gadgets. If you leave it on a bed it can easily fall down.