Is Mona a Mario character

Kinda, it depends on how far you go in considering what is a Mario game. The Mario series have many spin-off series and many series started off as Mario games. Some are the Yoshi's Island series, the Donkey Kong Country series, the Wario Land series, and the WarioWare series. Since Mona is in the WarioWare series, which was spun-off of Wario Land, which was a spin-off of Super Mario Bros, you could say that Mona is a Mario character.