Given that the two characters are very different in the way they view the world and in their view of propriety and communication, I would argue that they are equally at fault.
Aunt Alexandra has a preconceived notion of how little girls should act and Scout goes against so many of these notions that it is hard for Alexandra to see through those superficial differences and see what a smart and thoughtful and good girl Scout is. She is distracted by the superficial differences and this hinders their ability to communicate.
Scout too gets caught up in superficial things and refuses to try to understand why Alexandra acts the way she does and why she has the expectations of Scout that she does.
So the failings are not necessarily equal but they both share fault for the difficulties they have in communicating.