Ok this is one of smogon's weird things. They put Celebi in OU while Mew remains in UU. Now I have questions to ask...
They both have the same typing, except celebi is an extra grass. They also have the same BST
Has unnecessary grass typing which just gives it STAB, no support to its psychic typing, in fact it has a ****load of weaknesses. Though it has a nice resistance to water and ground...
Though it's typing is poor, it's not as bad as celebi's. Most notable fact is the movepool of this thing learning a WHOLE lot of moves. It can learn both amnesia, iron defense, though wallebi gets recover.
So Mew is as good as if not better than celebi. Now I wanna know why it was placed in UU