I have heard that Lincoln said that for most of my life. However, it just doesn't seem like Lincoln. When you stop and think, that quote is actually a put-down (or dissing as the younger generation would say) - a sarcastic slam in a way that lays the blame squarely on Mrs. Stowe's shoulders.
I just don't see Mr. Lincoln doing that to her face. Plus, he and everyone else knew that Uncle Tom's Cabin was just one log that went into the great fire.