Speaking as someone who loves the character platonically, unlike how Jason loves ferrets, I've enjoyed a few character design retrospectives and found
one about K. Rool recently. The design process for the King was to have a guy who was both at least a little intimidating, yet goofy and incompetent. Hence why he has meaty arms and royal attire, but is also a fat fuck and acts dumb as dirt, and has that fucked-up eye that pulls double-duty on both fronts.
As for why people might want to fuck him. If you really wanted to, you could separate pretty much any of those innocent-seeming traits and sexualize them. Literally any character you like WILL have rule 34 of them, and it's because you can break things down into fuckable chunks. Like so:
Meaty arms, dumb, yet confident? Himbo. Royalty? Women love power. Fat fuck? Deathfat orbiters. Kroc? Scaly furfags want him. Nintendo? Manchildren want him.
Seriously I wouldn't be surprised if there was rule 34 of the Aperture Science turrets from Portal. Go ahead, do a character design breakdown of that one and tell me there isn't a single trait to goon to.
So even though the character design process for King K. Rool had zero sexualization in mind, you still ended up with the Deathclaw effect. Despite everything, SOMEBODY will have the fat green fuck as part of their shrekxual awakening.
No wait, K. Rool debuted in the same game as Candy Kong though, so maybe they WERE on some gooner shit back then after all... I wonder if K. Rool's character designer also did the titty flower from Conker's Bad Fur Day.