It is most likely hacked.
It was possible to get an Imposter Ditto in Black/White 2 via the Dream World, and its owner could have just had the name Ditto. The shininess and the fact that it is level 100 however, hint that it is not legally obtained. If by "perfectly rounded stats" you meant perfect IVs, it is most definitely hacked, there is almost no chance of finding a shiny Ditto with 6 perfect IVs and its Hidden Ability. It doesn't cause your game any harm though, and there is still a(very small) chance it is legal. I see no problem in using itthough, unless it has illegal moves, Abilities, etc. as long as it doesn't have any of these traits it is still "legit".
Edit: It is hacked.
HA dittos were found in a certain hidden grotto, but they can't be shiny, so it is hacked
commented 4 minutes ago by The Trapinchinator
Thank you Quagmire.
Pokémon in the Hidden Grotto cannot be shiny and the contents are set when regenerated, they cannot be soft reset. Natures of the Pokémon, however, are soft-resetable
- Serebii
Rare case that there is full confirmation that a Pokémon is hacked, but as The Trapinchinator said Imposter Ditto are found in Hidden Grottos(one in Giant Chasm, if memory serves), and Pokémon found there cannot be shiny, meaning it is impossible to get a shiny Imposter Ditto in Generation V.
