This is a little silly to me, because there's fundamentally not much difference between any Mario Kart character other than weight class. And the Koopalings look a hell of a lot more different from each other than a bunch of Links. Not gonna try to convince you to like them but I feel like this is a moot point. Anyway,
I do think about this sometimes though, I don't really know how I feel about Bowser Jr's execution in Smash. I like him enough, I'm glad he's here and I'm glad the Koopalings were able to come along. But on paper, I fundamentally disagree with the choice. I've been vocally against putting alternate character skins on top of established characters because that will end up selling one or the other short. If you've went all in with one character, it makes it difficult to retroactively just slap a different model on top. It's why the ARMS thing would have never worked. But the reverse is you take away a bit of what makes the source character unique, in favor of boiling it down to the most basic depiction that can cover the most ground.
That's what Bowser Jr and the Koopalings are, but I get it. Essentially you're playing as the Clown Car. It is a bit odd when you resonate too much on how weird the Koopalings look outside of the clown car, or that they turn into Shadow Mario... these are inaccuracies that Smash usually tends to avoid. But I've noticed this is kind of just the Olimar / Alph & Pikmin situation 8 times over. I don't think I'd want them to remove the Koopalings in favor of strengthening Bowser Jr's character individually, now that they're here - I mean, they all do kinda act like Bowser Jr. But it does sell everyone a little short in the process.