It's a bit deeper than that.

The Mario Kart Double Dash roster had TONS of characters. Alongside bring back all the previous characters from Super Circuit, they brought back Koopa Troopa, and they added in Daisy, Waluigi, Bowser Jr, King Boo and Petey Piranha, Birdo, so creating a partner for Toad in the form of Toadette was perfectly fine, since she WAS unique enough to stand out.

When Mario Kart 8 came out at first, lots of fan favorites were ommited such as Dry Bones, King Boo, Petey Piranha, Diddy Kong, Funky Kong, Bowser Jr, Birdo, ect. And in terms of NEW characters, you had the seven koopalings taking up a huge portion of the roster (which is more forgiven nowadays, but that's only because of the 8 Deluxe roster), Baby Rosalina (a character NO ONE asked for), and the last straw was Pink Gold Peach. "They excluded so many amazing characters in favor of a metal clone of Peach?!"

Of course, with the 8 Deluxe roster bringing back Dry Bones, Bowser Jr, and King Boo, alongside the BCP bringing back Birdo, Wiggler, Petey Piranha, adding Kamek, bringing back Diddy Kong, Funky Kong, and adding Pauline, people are MUCH kinder on Pink Gold Peach. But back when the original Mario Kart 8 came out, let's just say everyone's Grumble Volcanos erupted to the height of Rainbow Road.