"She is a bad person."
Translation:Ona jest złym człowiekiem.
Well, they do mean exactly what you wrote, but the issue is more complicated. Generally, "osoba" seems to be used a lot less often in Polish than "person" is in English. I wrote everything that came to my mind about this issue here: https://www.duolingo.com/comment/19428542
But actually, it suits this sentence perfectly okay and is accepted: "Ona jest złą osobą".