Sadly, this seems to be the case. This is because whenever we see Marian interact with Mike, Lizzie or April, the easily predictable seems to completely baffle the Hell out of her. As an example of this tendency, look at how she reacts to Mike telling Lizzie that Elly had to move to Toronto because that was the only place she could find someone who looks like John. A normal person who remembered that children don't see the world the way adults do would smile a little because Mike is thinking the way an eight year old is supposed to. Since Marian isn't a normal person, she didn't smile behind her hand at the little kid logic like a normal person. She looked at us as if to say "Why would he even think that?! Doesn't he know that John and Elly weren't actually really destined to marry? Why isn't he thinking the way I do?"
Also, let's examine what happens when Lizzie tries to smuggle herself to Vancouver; a normal person would understand that her granddaughter likes being near her and doesn't want to say goodbye. Since Marian isn't normal, one of the things that preoccupied her mind on her death bed was "Why did Lizzie do that to my luggage all those years ago? I don't understand that and couldn't possibly see it coming."
This seems to be part of a wider pattern of Marian standing there looking baffled when confronted with the world around her. While she loved to talk about how on the ball she was, she clearly never had the least clue who the people around her were. As by way of example, she never seems to have realized what a terribly selfish and cruel person John is. More on that tomorrow.