In short; Your hacked Mew failed to pass some hack detection.
There are a limited number of sources for Mew, and other mythical legendaries that are distribution only, and most of them have tell-tale stats that can be checked against.
Namely, from what you've shown; the Original Trainer (OT) and ID No. do not match any distribution (There was a 2005 distribution that would have a non-predefined Trainer/ID, but that mew would be lvl 30+) and the Pokeball (either a quickball or regular pokeball) also causes it to not match any distribution.
There are other things that could be wrong with it: IVS, EVs, Encounter location, Game obtained, Ribbon...
Suffice it to say, if you used your cheating device to just encounter a mew (or any other distribution-only), and you catch it, it is probably not going to pass hack-detection.