The reason that this is a problem isn't just that her inability to see that clamping down on Elly in the name of a specious respectability while shrugging, smiling and refusing to see that most real men regard the momma's boy she blighted the world with with contempt ensured that her children fought like rats in a cage. The reason that it's a problem is that the same dimbulb who thought that a cranky child was the second coming of Hitler doesn't know that her daughter married a despicable imbecile.
The reason that I think that Marian has to have had a room temperature IQ in Celsius is that John is an amazingly easy person to size up. Anyone with a functioning brain would immediately see that John is a self-absorbed, verbally abusive, immature jerk who regards Elly as a resource to be exploited, a hindrance to be overcome and a child in need of guidance. While Jim seemed to be aware that John is a pinhead, he said nothing because Elly had convinced herself that she was happy with her asshole husband, hobby job and non-life. Marian cannot understand what makes people tick because she's denser than titanium and thus doesn't see that John needs to whiz on the electric fence yesterday.