You’re right. You’re absolutely right that people are judging the MIL and poking fun at her. I think one of the reasons is because people believe she again - is saying and doing things to hurt her DIL or another individual. Whoever was the aggressor, it seems even wrong to us because another person had to suffer the consequences of someone’s deliberately mean acts.
I have an issue with people who pass judgement on others because of personal sins they have committed. Now that they have done this, they are no longer paak-saaf Muslims like the rest of our community claims to be. Its not right. Its simply not right. None of us know where Allah swt’s mercy goes and whether someone is working hard to correct themselves. I know one girl personally who had such a shady past you would just wonder where her parents really were. Some might think she is going straight to Hell. Some might look at her life now and wonder if they have a different person.
Experience is the best teacher. Everyone learns their lesson at their own time.