who is the deaf daughter and why is it important?

Did you mean Jim's deaf daughter? Jim feels deep remorse for slapping her when she didn't respond to him. Jim harbours intense grief over this,

"the Lord God Amighty fogive po' ole Jim, kaze he never gwyne to fogive hisself as lon'g he live".

I think she serves to add more perspective to the character of Jim. We see Jim to be an incredibly caring father. Jim is much more than an escaped black slave. He has a life and his own history.