The Dreamy Sweater is one of the most sought-after clothing items IRL and for good reason. It looks adorable, snug, and cozy at the same time. It’s the absolute epitome of what a comfy sweater should look like.
The design for the Dreamy Sweater is copyrighted by Nintendo, so the only way you can buy it is from an individual seller on Etsy who has produced it as "fan creation". Technically these aren't legal for them to sell, but to Nintendo's and their lawyers' credit they're letting these listings stay up. It's a sign that they understand that these independent creations are a great way to keep Animal Crossing top of mind in their fans' minds (or that these sellers are just too small potatoes for them to worry about).