Well, they are a diverse cast. Just because they are all Koopalings doesn't mean they don't have their own individual looks and personalities. Hell, if we exclude babies and metal characters, there are 7 human characters on Mario Kart 8's roster.You just called the Koopalings diverse cast of characters lol...
And your examples are piss poor when you consider that there aren't 7 plumbers or 7 versions of toads.
And thank you for generalizing my statements as I would want generic enemies. I actually wanted Birdo (who has only been a 'generic' enemy in SMB2), Bowser Jr., and Diddy to still be in the game. Dry Bowser would also be cool to come back, much more visually distinct than 7 koopalings. Dixie, Cranky, E. Gadd, King Boo (was in Double Dash actually, forgot about that), and some WarioWare characters.
But you can go ahead and generalize my argument instead.
???

Seem pretty diverse in comparison.
I didn't generalize your statements. I referred to "people." If you want to lump yourself in with them that's your concern. Plenty of people wanting koopa troopa, goomba, dry bones, shy guy, lakitu, etc. and complaining about the Koopalings. Hell, even Yoshi, Birdo, and Toad are generic since they are more of a race of creatures than a specific character. Hell, a lot of these fan favorites like Birdo only gained their popularity through spin-off games like these but people won't even give the Koopalings a chance. The obvious best scenario would be "why not both?" but just because the Koopalings are in and others are not does not make the Koopalings bad.