Toadette requires an insane number of Toads to unlock in Super Mario Run making her the most difficult character to obtain in the game. You will need to get really good at Toad Rally and beat a lot of opponents to earn the 200 Red, 200 Blue, 200 Green, 200 Purple, and 200 Yellow Toads that are required to unlock her house. Once you have earned the correct amount of Toads, you can purchase Toadette's house in the the Special section of the Shop screen. Build the house in your Kingdom and she will be available for you to play.
How do I unlock Toadette in Super Mario Run?