to quote a pokedex Its DNA is said to contain the genetic codes of all Pokémon, so it can use all kinds of techniques.
meaning mew has arceus, and arceus was never proven a god it the event in HG/SS he made one of the creation trio with HELP from the Unown
I actually have developed a theory about that. I believe the Unown are Arceus's own energy manifested as Pokemon, which is why they are present when something like creating a Legendary Pokemon occurs, or in the anime when Dialga and Palkia first being fighting. Arceus is using the Unown's power (the power to warp reality) to create a new being without breeding, and possibly as the "DNA" of the universe since they are in the shape of letters and symbols. They serve to keep the order. The Unown are mysterious and have never served a real purpose, but Arceus is the only Pokemon to be linked to them.
Also, consider that Arceus has Multitype, which allows it to be any type. The Unown's only move is Hidden Power, a move that also can be any type. The Unown to me seem more like they hold the massive energy Arceus once had when the universe was created, which explains why it has BST of 720 when Mewtwo has 680. It's still higher, but not by much. Perhaps this serves as a way for Arceus to keep its own power in check while still remaining the most powerful (and allowing us to use such a potentially powerful Pokemon in battle without it being total overkill).
Mew however, other than learning every move thing and its Pokedex entry, has never shown traits of being the ancestor. It's never mentioned outside of the Pokedex and the anime never calls it that to my knowledge, as they only refered to it as the rarest of all Pokemon in Gen 1. The games seem to only imply it is an extremely rare Legendary Pokemon that MIGHT be the ancestor of Pokemon. (However, the Pokedex only states that "
it is said that Mew contains the DNA of all Pokemon", which is not stating it as a fact. If the games had expanded on Mew's origin more like they did Arceus, perhaps there may be a case, but right now I just see Mew as a really special and rare Pokemon, not the Pokemon ancestor.