It is pretty bizarre, but I guess I kinda expect Abby to be at least in shock by what Clarke did, if not outright angry with her. It's one thing for Abby to know in her head that Clarke saved Finn from horrific suffering, but in her heart, that's her baby girl turning into somebody that she doesn't know. You know what I mean? I can see her really struggling with it. And maybe a bit of it will be that Abby still can't get over what she did to Jake and now it probably feels like she's watching her precious daughter make the same "mistake" but, like, turned up to 11.
By the way, I think it's really cute that your mom cried without even watching the show. She must be really sweet.