The Smurfette Principle is basically a reference to franchises where being male is the default, and there are very few (often only one) female characters. This character usually has stereotypical feminine accessories, and has a personality mostly consisting simply of being a girl. Smurfette herself was the only female Smurf (well, most of the time; later seasons of the cartoon brought in a younger female Smurf named Sassette), although the other Smurfs are really only considered male by convention. They’re asexual, but their leader is named Papa, and many of them have masculine traits. Gargamel made Smurfette as a golem in order to spread discord in the Smurf ranks, and it worked for a while, as they were pretty much all taken with her immediately. She was only an occasional character in the original comics, but became a regular in the cartoon.
Other cartoons had variations on this basic idea, like Muppet Babies bringing in Scooter’s previously unseen twin sister Skeeter. Anyway, in the Mario universe, the character of Toadette seems to be an example of this trope. She was introduced in Mario Kart: Double Dash!!, and has made several appearances since then, but hasn’t been playable in any of the major games in the series.
Mind you, neither has Daisy or Waluigi. As I mentioned before, Rosalina has fared much better in this respect. I kind of wonder that they didn’t use her in either of the two most recent New Super Mario Bros. games, because I remember reading somewhere that they didn’t make Peach playable due to the fact that they weren’t sure how to animate her. I would imagine that a lot of this has to do with her long dress, and Toadette wears a short one. That would have given the games a playable female AND an established character rather than the generic Toads. Maybe they weren’t sure how her weird mushroom cap pigtail braids would move. The “ette” ending is not only feminine, but also diminutive, essentially making her Mushroom Girl instead of Mushroom Woman. From what I understand, the same is true of her Japanese name Kinopiko. The thing is, Toadette is by no means the first female Toad ever introduced. As Drew Mackie points out, The Great Mission to Rescue Princess Peach made all of its mushroom people female.
There are also several in the Mario role-playing games, and none of them have that weird mushroom-cap hair like Toadette does.
Instead, they appear to have more human-like hair growing under their caps, which fits with the occasional instances when Toad removed his own cap in the American cartoons.
Well, aside from this waitress, who has hair growing out the sides of her cap. I would imagine that required some rather painful piercings.
So why suddenly go with the cap-braids for Toadette, and why make her just a female counterpart for Toad when there had been other female Toads with more nuanced personalities?
Guess that explains it. Source: Pocket-Arsenal
For that matter, different sources identify Toad and Toadette as siblings, lovers, and friends. I just hope to the Mushroom Gods they aren’t all three.
Drew also mentions a few related items, like how it’s unclear why Toad came to be the name for the main Mushroom Retainer anyway. At this point, they were calling Peach Princess Toadstool in the English translation, and there are plenty of other mushroom-related things they could have called this character. That they went with Toad is even weirder considering the name was first used in a game where the main villain was a giant amphibian named Wart. Seems to me like characters named Toad and Wart would get along great.
While the Japanese games always used Kinopio as the name for both the individual and the species as a whole, it took a while before the English versions followed suit. In the manuals and cartoons, they were just called Mushroom People, which is a bit unwieldy but makes more sense.