Calling her "mentally challenged" is giving her too much credit, and an insult to the people who are actually mentally challenged.
True, but not being legally mentally challenged but still having low IQ/Intelligence/insert word for mental capacity, I've always had pity for them.
It's the lottery of life, people born in bad places, to bad people, to poor people, born with terible health, born with low mental faculties.
Your intelligence is mostly decided at birth, sure knowledge can help you wield that intelligence but it can't help you grow it, what you are born with is all that you get.
Pity those who lost the lottery.