Does the contrast between the dialects in "Sweat" undermine the author's attempt to present portrayals of African-Americans?
No, absolutely not. The dialect that Hurston uses is authentic. It only enhances the story. The language helps to reveal the true natures of the characters. Imagine if the characters spoke very proper English. This would not be true to their character, background, upbringing, which would be a tragedy. The protagonist is such a complex, rich character and to use any dialect other than her own would be a disservice to her and the story in general:
The dialect itself, aside from portraying the authentic speech patterns of Eatonville, allows Hurston to take ownership of the language. Like Delia, who assumes power through the story by shouting back at Sykes and insulting him, the author gains her own autonomy over the meaning of the text by putting it into the rhythm of her community’s speech pattern.