>>31641516last I checked, gen VII was the first time distributed pokemon were able to be nicknamed.
if it's an in-game event (which I don't think it is) then it's not able to legitimately be caught in a cherish ball.
if it's the FAL2010 mew I think it is, then it shouldn't be nicknamable unless:
A) your trainer name is FAL2010
B) your secret ID hit the jackpot and matched that of the event pokemon.
if I'm wrong, I'd love someone to correct me, but I can't think of how this'd work.