Outside of this really niche circumstance, Ditto is not very useful. Most of the time, it will be copying movesets that won't even hit the opponent hard, since most types are not very effective towards themselves.

Is Ditto worth keeping Pokemon go?

Ditto as a base Pokemon only knows how to Transform and Struggle, which shouldn't be a huge surprise. However, Ditto is cool because it will Transform into whatever Pokemon it's facing down. It will stay that way the whole battle and use that Pokemon's moves.

What are dittos good for?

Pokémon Go: Ditto's moves are unique

The first thing you need to know is precisely what the Transform ability copies from another Pokémon. Ditto will copy the moves, basic values and type of any Pokémon he targets with the Transform ability. Ditto also copies the CP of whatever Pokémon he targets.