She will not improve Obamas performance by much. And even if she does, that would be compensated by a lower Black turnout and margin. If this was 2000, she would get 20-23%, but after eight years of Obama - no, definitely not. People are overestimating Hillary's appeal among white Southerners and Appalachians. Hillary will court Black and Hispanic voters and not whites in MS. That alone will be a reason why she will do poorly.
She may win some white female votes that would normally either vote Republican or stay home. Her ability to win white Southern votes is very much overrated, however. She's not a Southerner and the white South is more Republican by the day.