No, Maledictuses are not immortal. In fact, even other blood curses can actually be the cause of a person's death, as in the case of Astoria Malfoy. As for Maledictuses, they are not immortal as is illustrated by the death of Nagini at the hands of Neville Longbottom. However, it is believed that after the complete transformation of a Maledictus into her animal form, she lives as long as she, as a human, would live and not up to the lifespan of the animal she became.