Do you have a photo? It might just be dirt or dust. Although I'm sure it must happen fairly often, I've never actually seen a dead or stuck pixel on a 3DS myself, whereas it's not at all uncommon for a small bit of debris to get trapped in between the touch screen and the LCD. It's also common for bits of dirt or debris to get stuck on the backlight for the LCD, which can also end up looking similar to a dead pixel.