[conistonwater]

It could also be the case that "mistakes were made" is poor, but without it having anything to do with it being in passive voice. Passive voice also doesn't necessarily "diminish the actor", consider "She was diagnosed by the doctors at X, and the diagnosis was later overturned by a senior doctor.". That's a kind of a blanket claim about passive that's just bound to be wrong when you check it on actual English sentences (I think Pullum's paper I linked to in another comment addresses this well).