Obliquity is a characteristic of being deliberately indirect or vague, especially to fool or deceive someone. The obliquity of many politicians makes it hard to get a straight answer from them.
If your teacher comments on the obliquity of your poem, it may be a compliment, or it may mean your writing is just hard to understand. And while obliquity isn't a bad quality in a poet, it's less desirable in a math tutor, or anyone who needs to be straightforward and clear. We can trace the origin of obliquity back to the Latin obliquus, "slanting, sidelong, or indirect."