The term "antagonist" simply means anyone who opposes the protagonist. Even if Dedede isn't necessarily "evil", it's clear that he doesn't like Kirby and wants him gone from Dream Land so he can pursue his gluttonous dreams of stealing food. He'll team up with Kirby if he thinks it'll save his planet and home, but otherwise, he'd gladly destroy Kirby if given the chance.
Meta Knight can be considered an antagonist in Super Star, but in general he's considered a rival. Unlike Dedede, he and Kirby share similar goals of stopping evil. They just approach justice in different ways, which causes them to clash from time to time.
But he only really opposes Kirby three that I can remember (when not possessed). The first being to steal the food, but the second time, he was actually the good guy. King's Revenge being an offshoot of the first time. Well, there's Gourmet Race, but that's more of a rivalry than straight up villainy. You also seem to think he's stuck back in Kirby's Dream Land when he really hasn't been keen on stealing all the food since the first game.
The villain argument is a little silly I feel. King Dedede is one of Nintendo's most iconic boss battles, alongside Bowser, Ridley, etc. The exact story nature of how and why he fights Kirby seems nitpicky.
It's not as bad though as people claiming Wolf is an antihero and not a villain, even though now his purely villainous appearances are the most numerous.
That may be, but being a boss and being a villain are not always the same. As I stated, he was only really a villain once. For that matter, Kirby lacks in notable villains since they've essentially turned Dedede into another hero and they keep getting new villains for each installment these days.
To be fair, there are about 3 (4 if you count 2 as unique) unique instances of Wolf showing up in Star Fox games, and in Assault he was more of an anti-hero while in Command
he generally filled an anti-hero role as well. And let's be clear, I don't think he's necessarily evil, just ambivalent with his morals doing whatever pays no matter what the job is or who's paying. Yeah, that's pretty scummy, but he tends to only do evil things when that's the job.