But in the anime, we had a few strong ones, or ones used great by the trainers.Mostly all Ditto are weak, shiny or not, to tell the truth.
Duplica used her Ditto and basically beat Ash's Bulbasaur back in the first season, and The champion of the Orange Islands' Ditto almost beat Ash's Pikachu, who is arguably his strongest Pokemon.
So I still think that this is the only time we have seen a "weak" Ditto.