Mew isn't a creator, he's just the missing link of all known Pokemon, most likely with the exception of Arceus. Of course, both of those backstories are possibly just legends to explain something that is seemingly unknown or paranormal. I mean, for Arceus to be "God" he sure is easy to catch, tame, and even receive multiples of to a trainer's will; plus, his legend only exists in the Sinnoh region so far.
Ditto also apparently is made up the DNA of all Pokemon, so does that make him come before Mew and Arceus? Pokedex entries are just flavor text, in general. And if Ditto is artificial as a Pokemon that implies someone has received the genes of both Arceus and Mew in the creation of the thing.