I've read that ピカドン (onomatopoeia for a flash and for a loud bang) is used for atomic bombs, but is it, or was it prior to Hiroshima, used to describe the flash of lightning and the sound of thunder?
Both ピカピカ and ドン/ドーン were (and still are) fairly common and generic onomatopoeias. ピカピカ existed before WWII (confirmed on 青空文庫).
If I understand correctly, their combination, ピカドン, specifically referred to atomic bombs. It originated from onomatopoeias, but ピカドン itself is a noun.