How is the death of Frankenstein's mother ironic?

The lovely Elizabeth, adopted by the Frankenstein family, contracted Scarlet Fever. Though Mrs. Frankenstein was made aware of the possible danger to her own health, "when she heard that her favourite was menaced," she continued to care for Elizabeth. Thus she contracted the Scarlet Fever and died while Elizabeth lived and went on to marry Victor only to be killed by the monster later in the novel.

You could also say that since Victor's creator died, he- as the creator of the monster- must also die.