I think it's time to throw some statistics in this discussion. VGChartz has listed sales for next to everyone game and I think you can trust the numbers they listed, but some data is incomplete. The sale numbers are divided into three regions: Japan, NA and others (Europe and Australia).
So, you can easy compare the Metroid series to the Kirby series.
The results:
Kirby series
Metroid series
Compare the sales of both series in NA and Japan (and ignore the VC-Games listed)
#1 Kirby games sell very well, better than Metroid games
#2 The Metroid games are strong in NA and Europe, but Kirby games are even better selling in these regions.
#3 Most important: Metroid games are very unpopular in Japan, because this genre is very unpopular in Japan. You can call the series unimportant and minor when looking at the sales of the series. The last game that sold well in Japan was Super Metroid for the SNES. That's 15 years ago.
#4 The Kirby series are very strong in Japan.
#5 Both series have a lot of recent games.
-> The Kirby series are by far more important than the Metroid series and the Star Fox series for Nintendo. It's not wrong having the Kirby series at #4 after Pokémon, the Mario series and the LoZ. 3 characters and 2 stages is a good representation for Kirby.
The SSE isn't. There's way too much Kirby representation in the SSE, all Kirby characters are very important, the series got much more representation than necessary.
As for the Kirby characters being too strong (-> bias):
I somehow think it could be biased, that they're too strong. It's unrealistic that Sora Ltd. didn't found the D3 CG/infinite (as example). The first videos of free play we've got from the Brawl demo before JPN-Release showed us that D3 could CG. Kirby needed a buff, he's a decent char and he's no way overpowered. Yeah, Meta Knight... I don't think I need to say something about him, it may bias that he is like in Brawl...
I think many people forget about Melee. Both SF-characters were top tier and Kirby was just horrbile. Not only as a character itself, the representation of the Kirby series was also horrible. In my opinion Melee-Kirby is the worst character for competitive play compared to the rooster in all three smash games and the nerf from SSB64-Kirby to Melee-Kirby was also the biggest one in smash's history.
What are all people saying that the SSE is like a Kirby game? The whole Smash series is a Kirby game. The reason why Smash is like Smash and not like other fighters is that HAL created the first Smash game and they took the basic mechanics from Kirby: Super Star (awesome game btw). Also most ideas introducted in the Smash series have their origin in the Kirby games.
That's my opinion. I thought it's time to register on Smashboards, because I'm reading it for years, but I never had the need to write something in this forum. My English is bad, because it's not my first language and I was very lazy in school.