I'm currently writing a master's thesis on the representation of the "Southern belle" onscreen. Now, I would like to hear some thoughts about this...

I don't necessarily find the Southern Belle trope to be racist; but it does depend upon what era of Southern Belle you're discussing. This cultural stereotype is found in history but also within contemporary culture. Cinematic depictions abound; perhaps one of the most famous is Scarlett O'Hara from Gone with the Wind. I am more inclined to call the Southern Belle something of a sexist stereotype, as opposed to misogynistic. Of course, all American culture was sexist during the Civil War area depicted in this film; but the Southern Belle attitude and demeanor found in contemporary cinema holds onto this dynamic of women being feminine and somewhat submissive to men (even if they are bold and strong-willed like Scarlett). The Southern Belle is not meant to depict women as weak or subservient, because these characters are usually shown to be enjoying their particular expression of gender roles.

There is often a tendency for these characters to be portrayed as extremely flirtatious and sexually manipulative, as well as ultra-feminine (what we sometimes call in more modern parlance "high maintenance" which refers to  high standards of grooming and dress that take great effort to maintain). We see this in Scarlett, particularly in her earlier scenes, when she refuses to eat before a picnic so she'll fit into her dress, when she pinches her cheeks to look as though she's applied rouge, and when she goes out of her way to dress in finery so she can impress Rhett Butler.