The inconsistency of genius is a consistent theme of creativity: Even those blessed with ridiculous talent still produce works of startling mediocrity. (The Beatles are the exception that proves the rule, although their subsequent solo careers prove that even Lennon and McCartney were fallible artists.) The larger point is that mere imagination is not enough, for even those with prodigious gifts must still be able to sort their best from their worst, sifting through the clutter to find what’s actually worthwhile.
Nietzsche stressed this point. As he observed in his 1878 book Human, All Too Human:
Artists have a vested interest in our believing in the flash of revelation, the so-called inspiration … shining down from heavens as a ray of grace. In reality, the imagination of the good artist or thinker produces continuously good, mediocre or bad things, but his judgment, trained and sharpened to a fine point, rejects, selects, connects…. All great artists and thinkers are great workers, indefatigable not only in inventing, but also in rejecting, sifting, transforming, ordering.
Notice the emphasis on rejection. Nietzsche eloquently describes the importance of not just creating but recognizing the value of what has been created.