In The Chrysalids, what is ironic about Sophie being discovered in a good season? What might have happened if the crops and newborn animals had...

Sophie’s discovery at that time is ironic because it came after David’s father made an announcement that they had defeated the “forces of Evil.”  If crops had been better she might not have been discovered because everyone would be too busy.

We are told that Waknuk is having a good season, which means that every animal born and crop raised looks like it is expected to.  There are no “mutations.”

The season was a good one, sunny, yet well watered so that even farmers had little to complain of other than the pressure to catch up with the work that the invasion had interrupted. Except among the sheep the average of Offences in the spring births had been quite unusually low. (Ch. 5)

If there had been more mutations, people might not have been on the lookout for some.  Since they had nothing else to do, Sophie became a top priority.  She had survived all of those years with her extra toes as her parents hid her.  Then one day she let her guard down and was spotted.  There are so few “condemnations” that David’s father announced that “that Waknuk would seem to be giving the forces of Evil quite a setback this year.”

David's father's comments are ironic because he doesn't realize the full significance of his words; exactly the opposite of what he expects happens.  David’s father announces that they have defeated the forces of evil because they have had no mutations, and then Sophie is discovered with her extra toes.  This is probably one of the reasons David’s father was so brutal to him about turning Sophie in.  He made a fool of his father.

In a way, Sophie’s discovery is linked to the good season because she and David explored more.  David does not have as much work to do. Even if they were careful, they were not careful enough.

With everyone so busy I was able to get away early, and during those long summer days Sophie and I roamed more widely than before, though we did our adventuring with caution, and kept it to little-used ways in order to avoid encounters. (Ch. 5) 

Unfortunately, David is not able to protect Sophie.  He tries to hold out long enough for her to get away, but his father eventually beats the information out of him.  Even though it was not his fault, David is guilt-ridden for having given her up.