It might be hacked.
Mainly because it has a Premier Ribbon, only given to event Pokemon, and no distributed event Mews have the OT 'Wi-Fi'. There were some events in Gen III where the Event Mew you receive is your OT, possibly meaning that the person who caught it had the OT 'Wi-Fi' in Gen III, but this is highly unlikely.
However, the ID No. '11219' was in fact used on an event Mew in Japan, 2009 and Japan, 2010. You received both at Level 5, and both of them had any nature.
Whatever you make of this, it has a Premier Ribbon, so you can't trade it over the GTS. But you can trade locally if you don't wish to keep and there is someone willing to take it.
Hope I helped. :)