I can actually see the Koopalings being his niece and nephews. For the number of them and the sheer variety of their appearances, this seems more realistic than the idea that they were all his work. Unless more than one mother was involved.
Although, with the type of cartoon rules that Mario's world obeys, Bowser having eight kids with instantly-recognizable outstanding features isn't far-fetched in itself. It makes Bowser Jr. the weird-looking one, for being so much like his father and sharing his name despite (until now) apparently being the youngest.
For what it's worth I've always for some reason had it in the back of my mind that the Koopalings WERE, in fact,
CREATED BY...DARK MAGIC
likely supplied by Kamek. I don't know where I got that idea but it's always at least been a "well maybe" for me.