Both Mallow and Steenee still barely have a character or involvement or anything and yet the latter is now gonna fully evolve. It's another Noivern situation where growing it is about the only thing they can think of it to do.
You are sadly right about this. Mallow and her Pokémon are probably the most underdeveloped characters of the S&M anime, or maybe even the whole Pokémon series as a whole.
If a Pokémon does evolve in the anime than it kinda has to have reasons for it. But Mallow and her Pokémon do barley anything in the anime, that I really can justified this evolution at all.
I think the problem with this is, that Mallow's character is just written as a nice girl. This is not an image that a writer can do much with. Each of the other classmates at least have other images to their personality. Lillie had PTSD and a rocky relationship with her family, Kiawe is a hot head and a farmer, Sophocles is a geek/nerd/inventor and not very fit in terms of sports, Ash can be a clumsy fool sometimes, has an interesting family-relationship with characters that he is not even related to and is very energetic and Lana can be sometimes a bit of a weird girl with hidden talents.
Poor Mallow has nothing really, or at least they do not play much with the very few talents that she has. Her cooking talents are not as much played out like the ones from Brock and Cilan and also her family relationships are not really that much shown compare to the other classmates.
And I think this is the reason why her Pokémon does evolve, because the writers kinda have to give her something.