Why is "proving your mother is your mother" considered a joke?

Because mom's words are long-winded, and then she will take care of your affairs everywhere. Mom is the person she trusts most and the closest person to her. No matter what harm she gets, she will return to her arms. After all, her mother is her own mother. After all, she gave birth to you and raised you, so don't think it's a joke. After all, she was raised by her own mother.