I think it's normal. Both my children's schools encourage them to draw. Maybe you can encourage her to write something with the drawing, if you worry about her ability in expressing herself in words. I know some schools allow children to drqw in composition if they don't know that particular word, that I don't think is appropriate.
Congradulations! You should encourage her to develope this interest. In James L. Adams' book "Conceptual Blockbusting - A Guide to Better Ideas", a visualiser is a better problem solver. Drawing is a good way to express her feelings and it will give you plenty of quiet time. So don't worry, as long as she can finish the homework quickly (not using drawing to delay her work).