The answer to this question is subjective and up to individual readers to state their opinion and defend. You may certainly claim that Mark Twain does a terrible job with creating Jim and that Jim is completely unrealistic. Personally, I would disagree with that statement. I am absolutely likely to give Twain the benefit of the doubt when it comes to explaining how a poorly educated slave like Jim might talk and act. Twain was born in 1835. Slavery was alive and well, and he would have been 30 by the time the Civil War came to a close. When Twain was a young boy and still living at home, he would have had opportunities for close observation of black slaves. His family owned slaves. He might be against slavery, but that doesn't mean he doesn't have close personal experience with how slaves act. Additionally, Twain based Huck Finn on a real person. It makes sense that Twain would have done the same thing with Jim. Perhaps Jim is a combination of several real individuals, or perhaps he exists as a representation of a larger group, but that makes Jim quite realistic in my opinion. In terms of how Twain conveys Jim's realism to readers, I feel that Twain is able to do that through using dialect. Jim's dialogue sounds like a poorly educated man.

"Well, it's a blame ridicklous way, en I doan' want to hear no mo' 'bout it. Dey ain' no sense in it."

Are you being asked to give your opinion regarding the effectiveness of Twain's writing in portraying Jim as a real person, or are you soliciting opinions from eNotes editors?

If you are supposed to be presenting your feelings, then you need to look for support for your opinions in the parts of the book that explain Jim's character, appearance, and actions. Do you identify with the reactions Jim had when he discovered Huck hadn't been drowned, when he was afraid of being discovered, when he was mourning being separated from his family? If you feel these were effectively written to create the impression of an actual person with emotions and feelings and motivations based on potentially real situations, then you would feel that Twain was successful in creating the character.