No. There are two reasonable explanations why Richard told Hurley not to listen to him.
1) Richard knows that MiB takes the form of people that are dead. Since Jacob is dead, Richard must think that MiB took the form of Jacob.
2) It could also be because Richard is now disillusioned with Jacob. He has been told all his life that he was instrumental in Jacob's plan, but told that he will only learn what that plan is "later". Now that Jacob is dead he felt that the plan was all for naught and his life was meaningless, so he may have told Hurley not to listen to Jacob for this reason.
MiB hasn't taken the form of Jacob. Hugo has a special ability to see dead people in a different way from when MiB has impersonated dead people in the past. Ilana also said that MiB is now "stuck this way", in Locke's body.