Mario Kart Wii is where the series really started to rely on archetypes way too much, most characters can be categorized into either:
Overall wearing men.
Princesses.
Babys.
Dinosaurs.
Toads.
Kongs.
Bowser & Family.
Mooks.
Or other alternate versions of existing characters.
Double Dash!! had an excuse as mentioned before, but in Wii it was clear that they weren't even trying and with Petey being sadly gone King Boo is the only one on the roster who doesn't fall into any of these types.
The worst part is that a character doesn't even need to be bad in order to be part of the problem, Funky sure as hell isn't a shit character, but then you look at the roster layout and yeah...
It doesn't help that the series has grown a habbit of creating bad "original" characters just for the sake of filling these archetypes, the only Mario Kart created chara that has any ground to stand on is Toadette, who while filling in a role that Toadsworth could have filled as well at least provides us a third distinct recurring a Toad character and a partner that helps putting "THE Toad" on a the pedestal as a main character he should always have and that's not even going into her making appearences in non-spin off games, Pinkgold Peach and Baby Daisy/Rosalina on the other hand are pure trash and only serve to make the rosters they are in even samier then they were before.
The series is in serious need for characters that don't fall into any archetype like E.Gadd, King K Rool, any given Wario/RPG character and maybe an retro oddball like Mouser or Foreman Spike, because as I said even good charas can make a roster feel samier when falling into a specific category, the Koopalings being a good example, as much as I love them and their inclusion in 8, I can't deny that having 7 out of 30 (not counting DLC charas) slots filled with the same type of character doesn't make for the most appealing looking roster.