On most airlines the upper deck was originally used as lounge space, and then later for first/business class, which may have helped with the poshness you experienced.
In the early-mid 2010s I was fortunate to fly several times on the upper deck of an EVA Air 747 (SEA–TPE). On these planes, the upper deck was regular economy in a 3x3 configuration. It felt like a miniature 737 inside the huge 747, cozy in its own strange way. They also provided slippers and the window seats had a fun shelf/storage space, making it feel all the more special.
In the early-mid 2010s I was fortunate to fly several times on the upper deck of an EVA Air 747 (SEA–TPE). On these planes, the upper deck was regular economy in a 3x3 configuration. It felt like a miniature 737 inside the huge 747, cozy in its own strange way. They also provided slippers and the window seats had a fun shelf/storage space, making it feel all the more special.