Why might it be construed as “poetic justice” that Victor’s worst catastrophe comes just as he is to be married?

Well, the thing that the creature wants most in the world is a companion. When he looks at Mrs. Frankenstein's picture and sees Justine Moritz's smile, he thinks that no one will ever smile at him that way because of the way his creator made him. He wants a female companion with whom he can run away from the world of humankind—someone who will accept him despite his appearance because she will look like him. He convinces Victor to create this female companion for him, and then, just as Victor is nearly finished, the creature watches as Victor destroys the female as a result of his own second thoughts. In other words, Victor denies his creature love and acceptance and the comfort of a companion, and so it seems only fair (to the creature, at least) that the creature would deny Victor those same things.