The one exception is Jim who everybody just casually dismisses, yet he's the one truly good hearted and upstanding character in the entire book. It's a lesson about the difference between what people pretend to be like, and what they are really like. Which makes the critiques of the book, often focusing on pejoratives towards Jim, just so unbelievably ironic. It's the deterioration of thinking in society.
Sure but you have to read the book first to know that, whereas the reader gets slapped in the face right away with the racial slur while reading the book for the first time, which is why some schools don't read Twain. This thread is about why people might not pick him to be the "America's Homer", I'm not trying to say anything more than that.