Mark Twain's viewpoint doesn't make Huck Finn not readable but it might mitigate his portrayal of the "Injun" in Tom Sawyer. But sure, lots of historical writers held viewpoints which would be pretty abhorrent today and it doesn't make their work unreadable.
I think it adds some fascinating insights into Huck Finn, Tom Sawyer and Roughing It. The last work is the one with the most racist content about Native Americans.
Part of what we learn from Twain's selective racism is how people influence each other. The characters in all three books were based on real people that Twain knew. Twain's autobiographies say more about it, but the thumbnail is there was a real slave, beloved by the neighborhood children, when Twain was growing up. Twain's experiences with this man basted his racism to atoms. A real world criminal who happened to have one Native American parent cemented Twain's racism towards Native Americans as firmly as it it were carved into the bedrock. In both cases, the racism of one man was profoundly influenced by the behavior of another.
This poses some disturbing questions about Card's childhood. In other writing Card equates homosexuality with pedophilia. His lackluster, disingenuous "defense" of his Hamlet rewrite is a clumsy effort to sidestep this issue. Why does Card have such animosity towards homosexuals and why is he so determined to connect it to pedophilia? Could this be mere cultural and religious conditioning, or is there a deeper, more personal cause? The shower scene in "Ender's Game" has the feel of a real incident that befell a child. In the book there's a clumsy victory for the hero, where he accidentally kills his assailant. This is a common fantasy for rape victims. The trauma is relived, but with a different outcome. Could Card be trying to tell us something about his own childhood? Having been raped by a pedophile when he was a youth himself would certainly cement a young man's hatred.
Was Orson Scott Card really molested when he was a child, or is he merely trying to give that impression with his writing?
And what of Rosencrantz and Guildenstern? In Card's version, Being raped as children turned both men into homosexuals. Could Card himself be struggling with homosexual desires, desperately trying to blame them upon the traumatic events of his own past?