Alright, then if no one else has have any issues, then we could argue the Smash Fighters, plus the bosses, to possibly scale higher, since Mewtwo is considered to be
arguably the strongest Pokémon in existence by Palutena. This matches with some of Mewtwo's trophy descriptions, saying that
it's battle abilities have been radically heightened and was designed to be "the ultimate Pokémon". (Also for some reason, it wouldn't let me copy a link to the highlighted parts I wanted to link to.)
This would imply that Mewtwo, and by proxy everyone else who's either playable or a boss could scale to Dialga, Palkia, and Arceus, which helps strengthens the idea that everyone scales to them, as the OP proposed. And seeing as Galeem canonically claps everyone with ease, including those three mons, it wouldn't really affect the scaling all that much.
To make a long story short: We could argue everyone is
3-A possibly
Low 2-C via Mewtwo.
Edit: I think I might've misread how Smash gets Low 2-C (Originally thought it was Arceus + the light and dark duo), so in case it's
ONLY Galeem and Dharkon who scales to Low 2-C, then it would just be 3-A.