I don't think that is a fair comparison. Zelda and Mario constantly change up styles much more than the Kirby series does; the Mario series for example has several pillars: the 3D games, the 2D ones, the RPGs which at the moment can be subdivided into the Paper Mario series and the Mario and Luigi series, and the spin offs, with Mario Kart being the most prominent. And the Pokemon series tends to introduce around a 100 new characters each generation.
Kirby, aside from a couple of spin offs, doesn't shake up it's style nor introduces that many characters which is the reason why it's games tend to have a much more consistent quality with the fans, that is because they are safer and much more simplistic. Which Kirby game would you consider the Mario64/Galaxy/Odyssey/OcarinaofTime/BreathoftheWild of the series? Games that turned heads and shook a lot of foundations at the time? While most of them are good, I say none.
That's why Kirby games are less divisive; you can expect them to be good most of the time without much to worry about but most of them aren't impactful either. With Kirby I see four constants: Kirby himself, Dedede, Meta Knight and Dreamland, toss in Bandana Dee once in a while, and that's what we almost always get on most of it's main iterations. Have these in Smash and you have represented most of the series. There are some new characters once in a while like Magolor, but the others are the core elements. In fact, Magolor is the only brand new thing that I can think off that was introduced in Returns to Dreamland, because the rest of it was a re thread of Super Star. And the Ultra Copy Ability introduced there was already added as Kirby's final smash.
Representation in Smash goes beyond characters. They do the best job of representing the overall series and I do agree that having Kirby, Meta Knight and King Dedede represents the series quite well from purely a character perspective. Bandana Dee would be nice but isn't quite as significant as the other 3. Stages are the next biggest since they show locations from the series and are where the music for the series comes into play along with other little references here and there. This is where the major representation issue is. It doesn't matter that Kirby's style overall stays rather similar. Dreamland 64 obviously doesn't reference anything new since it was made for Smash 64, neither does Green Greens or Fountain of Dreams. Halberd is simply a ship within the series that has made a number of appearances but to my recollection, the only time Kirby has ever been on the Halberd was Superstar during Meta Knight's Revenge. Then you get to the new stages from 4, the 3DS one flatout references numerous levels from the Gameboy games when it could've done that for any Kirby game and GCO is also from Superstar. Looking at characters and stages and not counting remakes of games, all of that Kirby representation is from 1996 or earlier. Assist Trophies are likely the next best representation and all Kirby has is Knuckle Joe and Nightmare which still originated from the older games. I don't care enough about stickers or trophies to look at what all Kirby has for them and as has been said, there's some songs from Triple Deluxe ported right over and Kirby got a new Final Smash and that's it for modern Kirby representation. Oh and the Dragoon is from Air Ride.
Meanwhile, Mario has characters and stages that have origin from the 2D platformers and 3D platformers both old and new, an Assist Trophy from the spinoffs and stages too, a stage from the RPG side and whatever else. Zelda has TP and WW representation through characters, add OoT to the mix thanks to the 3DS stage. Plenty of items with the Cucco, Beetle, Fairy, Skull Kid, Midna, Ghirahim, music from ALBW. Pokemon gets it easy thanks to the Poke Ball item but we have stages to reference both gen 5 and 6 along with music and Lucario is from gen 4. Worst representation probably goes to gen 2 and 3 though I'm not gonna bother to look at what all Pokemon are in the game since they're in the stages, Poke Balls, trophies, stickers in Brawl. Heck, even looking at Metroid, Samus got a redesign based on Other M and a whole stage from that game along with music. Starfox also got stages based on more recent games, rather than just sticking to 64.
Admittedly, I didn't bother to look at the representation that Smash Run offers so that could alleviate the issue and the only 'research' I did was seeing when Superstar came out. Design consistency is one thing but largely ignoring somewhere around 15 to 20 years of a series seems to me like clear pandering towards a specific time frame of said series.
This post is likely a mess since it's just the first thoughts that came to mind put in whatever order they appeared in so my apologies for the messiness and potential typos since I just got off work~