Why is mew ou?
Because it's better than Pokemon in UU and worse than Pokemon in Ubers.
Why is celebi UU?
>>29133271
Not learning Crabhammer keeps it out of ubers
Because it has a high enough usage rate to be considered "over used", and isn't strong enough to be considered Uber.
>>29135043
Because Grass/Psychic is one of the worst possible typings.
Because of its stats and movepool