When Edward says this, he is in no way being rude or trying to insult Bella. At this point in the story, both Edward and Bella's relationship is still very new and they are still coming to terms with their relationship.
When Edward tells Bella that she "isn't that creative", he is really saying that not even in her wildest of dreams could she have created something like this. In all honesty, could you??
Edward has been living for a century complacent with the fact that he may never have someone to share his "life" with. Now, not only has he found his soulmate, she is a human no less! For Bella, vampires have always been something that existed only books and horror movies. They were something to be feared. Now she is in love with one. Neither of them would have ever imagined that they would have been in a relationship with the other. No one's imagination is that good.