Ancestry does not equal creation.
Science says our ancestors were monkeys at one point. By your statement Monkeys created human kind. That is not true.
Mew is stated to be the Ancestor of all Pokemon simply because it can use all TMs. That is why it is considered to be the Ancestor. Also it having the DNA of Arceus doesn't necissarily mean that it came before Arceus.
Arceus' legend specifically says there was no other life before his appearence. He created Palkia, Dialga, Giratina, the world, and then the spirits. I don't honestly remember it saying he created everything else as the Spirits had a hand in creating various things as well. Mew, could have come hundreds of thousands of years after this point, but, like the fabled "Atlanean Gene" and other science fiction genes where decendants of long loss races are found, this does not mean that Mew had any direct contact with Arceus.
It is a Pokemon that has adapted in various environments and lived for untold amounts of time. Movie/Anime wise and Non-canon Game wise Legends can breed. Mew could have bred with hudreds of ancient Pokemon collecting DNA from all of them and ones that may have been direct offspring of Arceus and other Legends.
By Arceus' legend, he existed before everything else, so Canon proof wise, Arceus came first. If you choose to ignore Arceus' legend then there is no possible way to discern who came first as we do not live it the world of Pokemon so we cannot recieve all myths, legends, and scientific studies on those Pokemon.