This isn't surprising. There really isn't much room for Hillary to grow in Mississippi, since Obama won virtually all black voters there, and "white" is pretty much synonymous with "conservative Republican" in Mississippi.
Honestly, Trump doesn't have much room to grow among White voters either (non-college educated especially).
Poll does seem accurate though. The white proportion is somewhat overrepresented by about 5 points while blacks are underrepresented by about 4 points and other races by 1 point.