Symbolism, Imagery, Allegory
Ever heard a story where there's secretly a crazy woman living in the attic? It comes from Jane Eyre, where a guy keeps his possibly unstable wife locked away upstairs and pretends she doesn't exist—but it's been used over and over again since then.
The madwoman in the attic is most often used as a symbol of what people do to women whenever they're not meek and perfect and quiet, or as a way to constantly poke at another character's conscience. Artemis gives this symbol a little bit of a twist, though, by making that crazy woman his mother with a whole host of mental and emotional trauma-induced problems. She stays upstairs all the time, "hunched on the bed, her pale limbs glowing white in the gloom" (2.18). Instead of locking away a woman he can't control, Artemis locks away a woman who's supposed to be able to control him.
Which makes us wonder: Who's the mad person really? Artemis's mother certainly seems pretty batty, but that Artemis handles the situation with such ease—he doesn't seek any help whatsoever—suggests that there is something seriously off about the boy downstairs too. Since attics often symbolize brains and mental states, Artemis's mother—by exclusively existing in the upstairs of his house (and houses often symbolize bodies)—arguably represents his madness even more than her own.
The twists on the traditional madwoman symbol don't stop there, though, and unlike the original madwoman in the attic from Jane Eyre who burns down the house, Artemis's mother gets a happy ending with a magically-restored mind. Whereas traditionally the madwoman has to free herself, here the madwoman is liberated by Artemis—which further reinforces the idea that the madness she represents is his, instead of her own.
If you're not convinced, though, consider this:
The fact that he used his wish to heal his mother is not a sign of affection. He did it simply because the Social Services were already investigating his case, and it was only a matter of time before he was put into care. (Epilogue.2)
In other words, setting his mother right is a move Artemis makes for his own preservation—he literally quiets the madness to stay on course with his own plans. So instead of a loving act, then, we can see this as the continued work of a madman… er… madkid.