So, the latest Sun & Moon episode seems to confirm Mallow is the most useless member of the main cast. She isn't the main girl which Lillie is, and doesn't own a starter Pokemon unlike Lana. Heck, even Sophocles is getting more attention than her, and that's due to him owning Togedemaru.
People thought Mallow was going to get the Grassium Z, but Ash got it instead, which I can understand since he has a Grass-type with him in Rowlet. So sad because Mallow is probably one of the more tolerable characters in the Sun & Moon but it's clear the writers don't see any purpose with her.