Curtains can crease after washing them and, in the long run, once you hang them, but it's worth noting that it happens less when you opt for heavier fabrics. With some other materials, you can easily leave them that way: for example, linen is famous for being wrinkly. However, if you find it distracting, you can iron or steam your curtains to get rid of these creases (check their labels or manufacturer instructions first to avoid ruining them, though).