No, it is suggested that that's what the researchers know
; we know what kinds of stuff do they know
if such idea reached a Pokédex entry. Also it's pretty obvious that, for at least one big part of the biodiversity in the Pokéverse, evolution does not work the way it works here, although it seems to be exactly like here
for the other parts of the biodiversity.
Not that Mew can not be the ancestor of all Pokémon, but he can and could be something similar. One of the problems is, if Mew is the ancestor of all Pokémon, he shouldn't be able to Transform into Castform or Porygon, for example. As well as what kind of ("natural") evolutionary process would, and why, bring up Zigzagoon and Shinx from the same creature that should be able to transform into eg.: Groudon and Reshiram.