Mew's entry in the DPP pokedex is this:
"Because it can use all kinds of moves, many scientists believe MEW to be the ancestor of Pokémon."
Other pokedexes say that it has the DNA of every pokemon.
Arceus' entries talk about mythology (as do Dialga and Palkia) so you could probably think of it like Mew is real, the other stories are just legends.
But you could also look at it from a religious point of view, like Arceus was the original pokemon before time existed (like God), but most other pokemon are descended from Mew (like Adam & Eve).
I don't know, I don't really pay attention to the pokemon mythologies that much! :)