Technically the script was trying to do an approximation of African American Vernacular English (the AmEnglish varieties spoken by urban working class and middle class African Americans in the US); in this context "you is" is supposed to be a marker of the differences between spoken English between different classes, but "You is" isn't even the best representation of AAVE (which generally tends to lose "to be" in the present simple altogether - "You kind, you smart, you important").

Just to clarify, this is a vernacular - it's the way a group of people speak on their day-to-day lives, not Standard English grammar.

You can read more about this particular sentence in the context of The Help (and the politics of using dialects in such a charged way) here: http://dialectblog.com/2012/11/14/you-is-smart-dialect-gripes-about-the-help/ ;