What makes Caroline's death so ironic in terms of Victor is the fact that this death propelled Victor's curiosity to seek a means to create life. This death was difficult for Victor to accept, if he ever actually did. In response, as Victor left and went to school, he withdrew from social activity and varied studies. His purpose became singular and he began playing with dead body parts in an effort to create life.
Another way that this death is ironic has to do with the further deaths that follow. The creature that Frankenstein creates as a result of his mourning for his mother eventually destroys much of his family. The responsibility lies upon Victor who only acted in response to his mother's death.