I have only seen one person in my entire life that had a mouthwash decanter. It was a good friend of mine's very rich grandma. She had one in her guest bathroom and it probably hadn't been used in years. It was one of several guest bathrooms in her huge old house and it was immaculate.
I think most people just take a swig of mouthwash straight out of the bottle. Its just one of those things. Having a mouthwash decanter seems like its just an excuse to spend more money and be arbitrarily fancy, kind of like a golden toilet seat.