One point of clarification, contrary to popular belief, Frankenstein isnotthe name of the creature. Frankenstein was the name of the creator of the creature, Dr. Victor Frankenstein. The creature is never really given a name, hence the popular confusion.
The overall theme of the story is that man should not try to do God's jobs. Frankenstein violates this tenet, by trying to redefine death, and bring a dead corpse back to life. Dr. Frankenstein is successful, but the creature is so ugly that everyone flees from it, this leads the creature to frustration, and he takes it out on everything in his path leaving a trail of destruction. Dr. Frankenstein finally catches up with the creature, and they come to a conclusion that if Frankenstein would create a "wife" for the creature, it would no longer be lonely with a counterpart who does understand him, and is not put off by his ugliness. But while working on it, Dr. Frankenstein begins to have second thoughts about the potential consequences of this course of action, and ultimately breaks his promise to make a "wife" for the monster. The monster then goes on another rampage and Frankenstein goes after it, pursuing it even to his own death. When the creature realizes what has happened to his creator, he returns and weeps over his creator. Then heads to the North Pole with plans to immolate himself as punishment for contributing to the death of his creator.
Cloning is a way of doing exactly what Frankenstein sought to do, redefine the end of life, by recreating people who have already died. So it is logical that Mary Shelley would have been opposed to human cloning.