Unless that "stuff" is the main point of the whole novel she supposedly read.
Just because she read something and forgot does not mean shes dumb.. shes scared and trying to adapt.
She forgot the main focus of that novel. The only think that scared her so far was the duke, and her way of adapting simply consisted of chasing Ain to shower him with love. She didn't try to understand the new world, the House where she reincarnated into or the customs. Raising a kid without understanding how the world works and the dangers within it is dumb. People will try to kill that kid with poison, if Ain didn't continue on his own, she basically gave a weapon to the enemies that can kill him.
She has just forgot stuff dont think that makes her dumb at all.