Indeed, Uncle Tom was fictional character, but the book written by Stowe is credited with advancing the cause of the abolition of slavery. Lincoln is said to have said to Stowe when he met her that she was the little lady that caused the civil war! Uncle Tom's Cabin is also said to be one of the reasons for Abraham Lincoln to even be elected.
Sadly, Uncle Tom has become a pejorative name to call those in the black community who "try to be white" in American society. Unfortunately, those so using the name that way are often social failures themselves because of their anger.