C'est une sale personne.
The given translation was: "She is a bad person" which doesn't seem quite right. The better translation according to the 6th definition in the Petit Larousse compact 1997 would be "He/she is a disagreable/unlikable/unworthy person." The translation of "He/she is a bad person" would be "Il/elle est une personne mauvaise" or, in the negative "Il/elle n'est pas une bonne personne."
Il/Elle est un(e) XXX is not French, you have to say C'est un(e) XXX.