One way to interpret this quote, which comes after Edward has revealed his true nature, and that he used to watch Bella sleep, is that he is teasing her about the "ordinary" content of normal dreams. While she may have flights of fancy, she couldn't have actually thought that a real-life vampire could exist, much less fall in love with her. His choice of words:
I laid my head cautiously against his shoulder, breathing in the smell of his skin. "I was sure it was a dream."
"You're not that creative," he scoffed.
(Meyer, Twilight, Google Books)
--is not meant to be an insult, but can appear to be thoughtless on the surface. Instead, Edward is trying, in his sometimes clumsy way, to help Bella on her road to mental acceptance of the supernatural; she would normally have thought this sort of thing insane, but her "creative mind," rooted in the ordinary, has not been acclimated to the concept of vampires just yet. Instead, it is meant as playful banter between people who are romantically attracted.