The first person to witness the sea of clouds was almost certainly someone in Africa. There are several mountains on the continent where rare conditions allow clouds to form under the peaks. Of course, humans, neanderthals, or other hominins in the alps may well have been in place for that. However, if we're looking for places that absolutely, guaranteed had people above clouds in the premodern era, the Grand Canyon and the Colorado plateau provide proof. Structures like this modern one would certainly have witnessed the spectacular weather in the canyon numerous times in their histories. It's impossible to know who was truly first, but we do know it happened a long time ago.