The correct response to how the sentence "Where was Mom anyway, and wasn’t there something she had to do?" shows irony is:
The reader already knows that Matilda forgot to turn the oven off.
This is ironic because while Matilda is questioning where her mother is and what she should be doing, the reader knows that her neglect led to a significant problem (the smoke and the fire) that has already begun to unfold. The irony lies in Matilda's carefree attitude in the face of the chaos she created.