I think the OP has the experience to differentiate between someone who's truly unable to comprehend and someone who's just uneducated.
She (that mother) reminds me of someone I met when I was in London touring St. Paul's Cathedral. She was from New England (I won't say what state, as I'm quite sure she was NOT a representation of what most people there are like

) Anyway, the tour guide was showing us places where the cathedral was damaged during WWII and told how members of the congregation actually got on the roof during attacks and knocked off incindiary bombs before they could set the building on fire. The woman was just shocked and blurted out (not once, but over and over, "Why would they bomb a CHURCH? I just don't understand why anyone would bomb a church!" Well, the poor guide tried to explain it to her, but she could couldn't believe that anyone would actually drop bombs on a place of worship.

Her husband tried to shush her, but she kept on muttering to him, "Honey, why would anyone bomb a church?" I did not get the idea that she was low functioning. I did get the idea that she had likely been one of those girls in high school that are too busy primping and flirting in class to listen to all the boring icky stuff about war and death.
Laurie, who can't wait to hear how the principal reacts in the meeting!