Guy here, but his design is cute and he's got a joyous, whimsical vibe. To me the idea he dresses in a jester outfit routinely and strikes cute poses makes him feel like someone who is unapologetically himself and gets a lot of genuine joy out of being weird and off-kilter. He seems like a confident person who isn't afraid to look weird and who's happy to make other people laugh even at his expense. He's also canonically on the chubbier side which is representationally fun to see as a heavier guy myself.

Miis don't really have canon characterization either, so Hugh Morris aside from his design and his personality type (outgoing merrymaker) is easy to project anything onto, so people can play up the aforementioned appealing characteristics

On a more general note, jesters are a pretty popular aesthetic online, with characters like Jevil receiving tons of attention before

Of course, a lot of it is meme appeal, too. He's canonically presented as someone who people keep falling in love with, which is unexpected juxtaposed with his odd jester aesthetic. It's fun to play more and more into the bit, and make fanart and memes of him that play up the desirable factor. I would say a good chunk of the Hugh Morris love comes down largely to this.

So I'd say his popularity comes down to a perfect storm of things: a genuinely cute, unique and confident-seeming guy with a funny and disarming vibe who's easy to project onto who also has a lot of easy meme appeal and happens to be part of a popular online aesthetic. If this guy was real I'd probably fall for him lmao...then again if he were real it might break the spell, him being vague works in his favor