Mrs. Merriweather is a member of the missionary circle, as are Aunt Alexandra and Miss Maudie. The meeting this month was at the Finch home, and the topic of the meeting was the plight of the Mrunas--a rustic tribe which lived in huts halfway around the world in Africa. The ladies, and Mrs. Merriweather, in particular, seem to have a huge sense of compassion for these poor, un-gospeled people whom they have never met.
Also at the meeting, though, this gathering of women concerns itself with the gossip of the day--and of course that means the fallout from the trial. Grace Merriweather, who is concerned for these stangers in a strange land, is throughly disgusted at the reaction of the blacks in town after Tom's guilty verdict in court. She says:
"...I tell you there's nothing more distracting than a sulky darky. Their mouths go down to here. Just ruins your day to have one of 'em in the kitchen."
The irony is that she is perfectly willing to show all manner of grace to a group of black people in Africa whom she has never met and has no relationship with, yet she is intolerant of the black community in her own town--and particularly her own kitchen--with her cook who has every right to be distraught at the injustice which has been done in Tom's case.
This double standard can be seen when she said of hypocrisy:
"At least we don't have that sin on our shoulders down here.... Down here we just say you live your way and we'll live ours."
She apparently has no idea how hypocritical and judgmental she is being--which is, after all, typical of that particular vice.
To Kill a Mockingbird is a story of many things, and hypocrisy is certainly one of them--and it shows up pointedly in this scene.