A lot of people wish that more of Kirby's Copy abilities were integrated into his moveset, that he's not well represented without them. The entire crux of their argument is that it assumes it's the Copy Abilities that solely define Kirby, and frankly, that's just not true. Kirby does not need a bunch of random powers sprinkled across his moveset to make him better represented, in all due honesty, it'd make him more cluttered, which might be fine for some Smash fighters, but probably not for Kirby specifically. There's more to simplicity than just being easy to use, the visual aspect is also very important.
Folks also assume that Sakurai has this huge bias towards the games that he made, and chooses to ignore the newer ones.
In reality, the newer games are a big fan of the older games. There's nothing that the newer games add and keep that wasn't already in those older games. There really is not that big of a dissonance between Classic Kirby and Modern Kirby.
They actually have the same problem that people have with the Pokémon games.
Newer games just keep it safe, while adding a new game specific gimmick that gets dropped by the next game. Though I'd argue that Kirby is a bigger offender of this, because they play it MUCH safer. Heck, pretty much every game starts you in Green Greens or some type of version of Green Greens. Imagine how people would react to Pokémon if every game's first route was identical to the starting routes and gyms in Generation 1. Also look at how tired people are of Sonic games that always start with Green Hill zone.
This is actually a common criticism that I've seen people make with the newer Kirby titles, with Star Allies being the de-facto example, and do I very much agree with that standing. Now this doesn't mean that they're bad, Kirby games are always very solid, after all if it's not broken, don't fix it. But a little innovation never hurts anyone, cuz otherwise it becomes stale. What this also means though is that in the context of adding anything worth while to Smash, there just isn't anything.
Followers on twitter have been pointing out that this doesn't only apply to the playable characters though, the Stages, music remixes, and other stuff also suffer, but I still don't think that holds water. I don't think there's any Stage themes that's solely in the newer games that wasn't also in the older games as well.
You can argue that the selection of Kirby stages is shallow, we really don't need three versions of Green Greens, and I do agree, but I chalk that up more to unfortunate planning. All three of those stages were each made for their respective Smash games. There's of course 64, the remixed version of it for Melee, which is what most 64 stages got, then a fun little Gameboy call back stage for Smash 4 3DS.
Ultimate brought back all but 15 stages from the prior 5 games, and when you do something like that, there's bound to be some redundancy across stages, and this doesn't only apply to Kirby unfortunately, multiple franchise stages also had this issue. But don't think of that as an excuse, but a criticism, despite having the widest selection of stages, pulling stages from past games is bound to lead to over lap. So that's less and issue for Kirby and more for Smash Ultimate as a whole.
And music is a different issue, because Sakurai doesn't choose what music get remixed, he gives creative liberty to the music composers, and THEY choose what songs they wish to remix. So the older Kirby themes getting remixed shows more favoritism from the music composers, not Sakurai. Plus it also doesn't help that most tracks in the modern Kirby games are just remixes of older Kirby music to begin with.
Again, the newer games are huge fans of the older games.
Overall, I don't really have the same issues that a lot of people have with Kirby in Smash. If anything I have more issues with the Kirby series itself, despite the fact that I do still have fun with them. Does that make their opposing view points from mine on the matter in-valid? By no means. If you think that Kirby could use an over haul in Smash, then cool! I'm sure that there are a ton of neat ideas that could be applied to him! But that's just not for me. I like Kirby as is, and I really don't think he needs that much.