Not legit. The lowest level possible for Mew in Diamond legitimately is level 30, which is the Mew received from the Faraway Island in Emerald, which is not possible anyway seeing as it was found in Kanto. So the only way it could have been from Kanto is if it was traded from HG/SS which has no legit Mew encounter-able.
The fact it is caught in a Master Ball makes this even more obvious as most people who hack or use cheats to obtain Pokemon use Master Balls. Also, Pound is legit at level 3, however Mew should also know Reflect Type and Transform at that level, so a Mew without those moves at level 3 is not legit if you do not count the move deleter.