Strange.
MK and DDD are characters Sakurai himself personally created, and judging by the new final smashes, it's not like he forgot about what they're about.
Perhaps Sakurai just... likes how he made them in Smash and prefers it over how he made them in Kirby.
The thing is that the movesets that they ended up with do feel right visually, and they do a good job at portraying the character's personalities. It just doesn't feel good to play. I should note that King Dedede's moveset inspiration is mostly from his boss encounters, with Jet Hammer likely also taking a page from Hammer Flip in terms of mechanics (Jet Hammer was an actual move he used in Kirby Super Star Ultra). King Dedede does have a lot of made up attacks as well, but it was likely more purposeful.
The problem with him is that, I believe, he's supposed to be a mix between a heavy hitter, and a swordfighter style zoner, but with more emphasis on the former than the latter like with Ike. This means that he has big buttons, but they are too slow to actually give him an effective range, so his gameplan just became spam Waddle Dee Toss and Chain Grap people to death, and then because that was a pretty silly (and unintentional) gameplan, it then became just Gordos in the neutral, which is not that great an option. His win condition seems to be Gordo ledge traps, which is actually really good, but it's difficult for that to be successful when he has trouble winning neutral so he can put his opponents in a ledge trap situation.
I feel like the dynamic when fighting King Dedede should be that you
have to respect his buttons, and then punish when he overextends since that's kind of how Kirby bosses work. I'm not entirely sure how you'd achieve that effect though. I feel like Meta Knight took this direction, but he ended up just being able to fly around everywhere and do what he wanted in the right hands, and now just seems...off. I think a swordsman that trades range for agility is a pretty decent angle for him, but I think he needs to be rethought at least a bit to pull that off. Also, none of his attacks feel particularly impactful, and I wonder if that's the sound design's fault. I love the Galaxia's unique sword swing sounds, but if they aren't selling the power behind them I think they might need to record better ones.