Actually, ditto tend to be some of the best parents in the pokémon kingdom. Ditto are often either eager to please or eager to maintain their disguise, and as such, they’ll work hard to resemble whatever their target pokémon may be. This is relevant to breeding because as a result, a ditto will often do its best to mimic typical brooding behaviors of whatever form it’s assumed to produce offspring. If such an example is not readily handy (for example, if it assumes the form of a male charmander to produce charmander eggs, but another male charmander isn’t readily available to mimic in terms of behavior), then it will simply do its best to serve as a parent to its brood however it thinks a parent of that species should be until their children are capable of fending for themselves. Otherwise, it risks breaking its facade.
It only really becomes a bad parent if it’s forced to produce eggs of a wide variety of species in a short amount of time. If this is the case, then it becomes confused and incapable of taking care of any of its offspring, so it will consequently reject the whole brood. Likewise, ditto used with an abundance of different partners will find it more difficult to become emotionally attached to any particular species and will begin acting detached from the act of breeding in a way. It will, in other, admittedly human terms, begin seeing breeding as a job and will begin to anticipate its movement to another partner, rather than anticipate its parenthood.
So in short, breeders: please don’t abuse your ditto.
The Pokédex According to Bill 