I don't think it's a stated goal anywhere that Kirby spinoffs are meant to evolve platforming as a whole. I do feel they are interesting ideas HAL has that probably wouldn't get published if it weren't for Kirby (or at least as easily.) Lets face it, developers are scared of risk and although HAL is more than capable of shooting out original IPs all of which could "evolve platforming" on their own without needing Kirby in them, it's riskier. When people see HAL they expect Kirby and when people see Nintendo they expect one of their major icons. It's why Feel Good ended up making a Kirby game and now a Yarn Yoshi. There's no denying this fact, that Nintendo puts their mascots on new games ideas they are wary of.
Kirby as a figure isn't actually needed to "evolve" platforming, because he is merely an avatar. A ball character could be designed differently all together and placed in a Canvas Curse-like title or other little creatures could be put in Mass Attack and it would not change what is actually "evolving" platforming... and that's the gameplay itself.
I'm not convinced that is the goal of Kirby or part of his Identity. It's more HAL rationalizing this aspect of the industry and using a mascot they made famous to sell other ideas. It also doesn't negate the fact that these diversions aren't polishing Kirby's main abilities and evolving those. Galaxy as a pure Mario game, with great ideas, didn't need to sacrifice Mario's main abilities and gameplay to achieve greatness. I would consider Galaxy more an evolution to platforming as a whole than even the recent Kirby spinoffs.
But there's a difference between evolving and innovating. Kirby spin-off doesn't evolve anything, they innovate.
Evolving means improving something existing, while innovating means bringing something new and fresh.
I do like very much the experimental side of the Kirby franchise because I see lots of innovative ideas coming out from it (you can say, for example, that Mass Attack is inpired by Pikmin or Lemmings, but those games are still very different, and there isn't any game that plays similar to it).
Even Rainbow Curse that's not very innovative, it still has those gondola levels that are very creative and fresh.
As I said in my previous post, I'm fine if they do new IPs, but at least they should keep the level of attention to details and quality at the same value of their classic franchise.
When Game Freak made Drill Dozer or Harmo Knight, they made something new, and they at the same time built up an interesting world, kept the level of details and quality high, and made it fun. Hal, making Boxboy, sacrificed a lot of things, such as depth, visuals, details, interesting world,etc. making it like any of those indy games that tries to emulate the 8 bit times (there is plenty of them).
And Mario Galaxy is subjective I think, as I think it sacrificed lots of things, the most noticeable are the flow and the level design (lots of puzzle solving, collecting 5 things killing enemies or solving puzzles, and automated flying sequences instead of actual platforming), but also the environment, that aside from rare cases like Gusty Garden Galaxy, is mostly generic space and little planetoids with no identity. And I think they (Galaxy 1 and 2) are the most boring Mario games, the ones with the lowest replay value of all them, try to replay them again if you don't trust... they are those kind of games that are made so you are surprised the first time you play them, because of some gimmicks, but those gimmicks are so forced and slow, without any freedom and little interaction, that replaying the level again makes me yawn instantly; kinda like the super abilities/hypernova in Kirby games.
(also, while badly executed in the past, the idea is not original at all, since there is a level of Sonic Adventure 2 that's just the same concept of Mario Galaxy, if you replace the launch stars with rockets and the lumas with help monitors).
If you ask me, the actual Mario games (3D Land and 3D World) are infinitely superior, and I personally prefer Captain Toad even more, because it shown how a very simple gameplay can be expanded to become a full game with awesome concepts, variety, creativity, innovation and some epic moments.