Gonzo The Great
Banned
You're counting the GBA and GBC ports as well as the Land games? Also, in three of the Kirby games you listed, Meta Knight is playable. He's also playable in Canvas Curse and in Triple Deluxe,Dark Meta Knight is a secret boss.
I counted Nightmare in Dream Land and Super Star Ultra, so why not? Both franchises have a history of getting ports, and they definitely matter if we're talking about exposure. Many detractors argue that kids today have no idea who K. Rool is, all while ignoring the GBA ports of the DKC trilogy and Virtual Console re-releases. Also, the Land games have completely different levels and plots, so I fail to see why they shouldn't count. And I'm not sure why you're mentioning Canvas Curse, since it's no more of a "platformer" than King of Swing.
I'd argue that Donkey Kong Land has the greatest plot in video game history. It's so Rare.
According to the game's instruction booklet, the story takes place after the events of Donkey Kong Country, where Donkey Kong and Diddy Kong are discussing about their latest adventure to Cranky Kong. Cranky admits that the adventure was more successful than he thought, although claiming that it was only because of the "fancy graphics" and that "Kids'll buy anything nowadays". After Cranky goes on about the games of his time, the two other Kongs try to convince him that it was not only fancy graphics that made Donkey Kong Country great, but that it "was just plain fun". However, this did not convince the old ape, so he challenges Donkey and Diddy to an adventure on an 8-bit black/white system (Game Boy), with King K. Rool stealing the Banana Hoard again. Once the bet is decided, Cranky Kong arranges everything and calls King K. Rool to come and take the hoard overnight.
I don't think "Meta Knight has been playable before" is a valid counterargument, because, unless a franchise receives multiple spin-off games, chances are that the villains will never have an opportunity to be playable outside of Smash. But if it really matters, then K. Rool was a playable character in DK King of Swing, Donkey Kong Barrel Blast, and Mario Super Sluggers. He was also going to be playable in Diddy Kong Pilot, but again, the buyout completely screwed everything over.
I don't know why everything has to come back to K. Rool. Meta Knight is an independently deserving character. Just because K. Rool isn't playable doesn't mean Meta Knight shouldn't be. They have nothing to do with each other.
It works both ways. K. Rool and Meta Knight are both iconic Nintendo characters who deserve to be playable. I'm not sure why decade long absences are okay for Meta Knight but they're not okay for K. Rool because he's ugly.