I think there are some reasons why Jim isn’t very famous name.
One is because of Huckleberry Finn and Tom Sawyer is name of story. So people have some chances to hear their name.
Second is that Huck & Tom are boys. So if readers are boys or girls, they may be able to be Huck or Tom when they read the story. Or they may be able to be Huck & Tom’s friends. And they had adventures with them. But Jim is older than readers. So people couldn’t to be him when they read the story, I think.
Third is because of Jim is a slave. I got some information about slave from the story of “Uncle Tom’s Cabin”. In this story (“Huckleberry Finn”), Jim’s master was not a bad person. But I don’t have a good image for slave. People worked very hard. But they couldn’t get any money. And they didn’t have any freedom. I think Jim is a good person, but I don’t want to be a slave. Some readers may think so, too.
Fourth is Huck and Tom are active and positive person. So I think they are attractive. They are admired by readers.
Posted for Yuri
By these reasons, Jim isn't a very famous name, I think.