I think I can safely say that does not happen, story progression at various times has turned members of the evil Shallow family into mermaids and none of them turned good, and this is over several different games.
I think it has more to with being given bad mermadic kelp, only the evil mermaids like Salty Seaworth do that, so I guess it needs somebody for that job, which is pretty pointless in worlds with no dive spots as you can't ask for it anyway
Still I'd take evil over mean spirited, hot headed or Diva any day, at least it doesn't change them to one of those traits.