It's hard to define whether Mew is OU or Uber. I suppose all the previous movesets it had from past Gens work. Both the losses of being able to Baton Pass Block and Explosion are sad. As for changed Sleep mechanics and Magic Coat's buffing, that's debatable.
If Mew is OU, it does have a niche to be a stall/wallbreaker. This can be achieved with an array of Softboiled, Taunt, Substitute, and Seismic Toss.
Mew is also notable for its ability to Pass Magnet Rise.