Darcy's not the most intelligent person from a pure 'dnd stat intelligence' standpoint. Her memory can be flaky and is often very subjective, she has little patience for difficult reading and sucks at maths and science. Academically she was struggling at school back home, and only wasn't forced to repeat a year due to the special consideration of being her mother's primary carer.

That said, when Darcy has a strong interest in something, if it's useful, or if there's an answer she wants to find out, she ends up putting the full force of her stubbornness at it and will generally keep at it until she gets it. This is how she ended up struggling through Renaissance fencing manuals and theological texts.

Additionally, while Darcy struggles with big-picture and long-term thinking, her short term wits are remarkably good. Back home during ghost hunts, she had a talent for finding creative ways to escape or outwit ghosts. The down-side is this lack of long-term thinking frequently meant she then had to deal with the consequences of her actions. Ask her about that one time she exploded a boiler to get out of a haunted basement, she loves telling that one.