A friend's artwork doesn't appeal to your aesthetic sensibilities, but they're absolutely in love with it—should you let them know that they're fooling themselves? Nope—the stars say that something as personal as a creative work cannot be objectively judged. It's all subjective, and cannot be graded in any way. You don't have to lie and say you love it, but don't hurt them by saying that you think it isn't good. Just be supportive about the fact that they're creating something.