This should not be a surprise to anyone. She had to work at being that stupid. Being stupid is, as it has been for a long time, hip, in that age group. Learning is not fashionable.
I graduated HS in 1974, and remembering some of my classmates, they were the same way. They had the basic equipment to absorb the education being offered, but they chose not to. In other words, their brain cells were functional, they just chose not to use them.
The folks that chose to learn what was offered were, as often as not, ostracized and made fun of.