Many are familiar with the the derogatory meaning within African American culture of "Uncle Tom." Typically this label is given to self-hating and self-denigrating blacks who seek the affirmation and benevolent praise of normative white society at their own expense, and the expense of the race. Extreme examples of such behavior can be found in the slaves who betrayed the insurrections of Gabriel or Denmark Vesey, or in a more contemporary context, Justice Clarence Thomas fits the bill for all practical purposes (as well as Ward Connerly, and several others).
Regardless, the character "Uncle Tom" in Harriet Beecher Stowe's novel, Uncle Tom's Cabin was anything but self deprecating. Actually he sacrifices himself for the sake of other slaves. Moreover, his uncompromising Christian beliefs in the face of brutal treatment made him a hero to white audiences. For them, he was the personification of Christian humility.
Tom's tormenter in the book, the brutal and savage Simon Legree, the Northern slave-dealer turned plantation owner, enraged white readers with his cruelty in stark contrast. Stowe convinced her readers that the peculiar institution of slavery was evil, because it supported people like Legree and enslaved people like Uncle Tom. Her book in its own way brought more whites to the cause of anti-slavery, but not to abolition. You must remember that "anti-slavery" not the same thing as abolition.
Now at the time of publication, in 1852, there existed a small number of persons in the United States who were literate. Of the that literate populous, one would need to interrogate the critical mass of transplanted Africans and free people of color who could read. Frederick Doulgass, the noted abolitionist and patriot, praised the book in his writings. He also was a friend of Stowe's who she consulted on portions of the text. There were those who found the novel condescending not just with the passiveness of Uncle Tom, but also the fact that the characters in Stowe's novel emigrate to Africa, they are never able to live in America. Again anti-slavery does not equal abolition.
So the question remains, just when did Uncle Tom become a bad thing to be? I can see the negative aspects of him being labeled weak and socially impotent in the face of oppression, but I seriously doubt that either Thomas or Connerly would have the intestinal fortitude to sacrifice themselves for the cause of freedom.