If it's from Emerald, and not ORAS, then it could be legit. It is actually possible, although extremely rare, to have a Shiny Mew from the Japanese version of Emerald, since shinylocks weren't a thing back then. Bear in mind, however, that the only way to legitimately get Mew in Emerald is if you have the Old Sea Map, which was only officially distributed in Japan back in 2005. Double that with the fact that shinyhunting in Emerald is more difficult than other games due to its RNG's fixed seed (more info if you're interested). The only way this Mew is legit is if someone managed to get the Old Sea Map from the event back in 2005, and knew how to subvert Emerald's RNG issues. And after all that, then be willing to trade it away. I can safely say that it's probably 98% just outright hacked, 1.9999% cloned from a legit Mew, which would still make it illegit.