It was a dream, per what's written, and I don't have any cause to doubt her word.
She doesn't say, don't condemn Jesus. She says, have nothing to do with Him - Matthew puts this in context of Pilate sitting as judge. It is reasonable that she meant then, do not have anything to do with judging Him, whether to condemn Him or release Him. Immediately before this one has the impression Pilate meant to dismiss the charges, knowing that He has been arrested for envy, not for any crime - but then hearing from his wife he puts the decision in the hands of the Jews, washing his own.
Given these things i have no cause to believe her dream was anything but a vision from God, putting the bloodguilt of God on the Jewish people, rather than on Rome, just as scripture had said, "Me, whom THEY have pierced"