I believe the missionary ladies are totally serious in their concern for the "Mrunas". As Christians, they are taught to be concerned about the salvation of non-Christians and the safety of Christian converts. However, they live in a society that sees Africa and the African-Americans who live in their own community as two distinct groups. It is easy to care for a people who live so far away. It is much more difficult to go against long held beliefs about racial and social superiority. The African-Americans that live in Maycomb are, for the most part, uneducated, former slaves. The Mrunas are an exotic, far away people that probably seem like "noble savages" to the women of the small town. That, of course, is the irony that Harper Lee wants to show.
I'm sure the ladies thought that they were sincere; it just wasn't the most effective form of compassion. Mrs. Merriweather, when talking about them, was crying: "Oh child, those poor Mrunas...the poverty...the darkness...the immorality...I made a pledge in my heart. I said to myself, when I go home I'm going to give a course on the Mrunas." She seems sincere enough; weeping and pledging. Too bad she spends the rest of the time back-biting and subversively deriding almost everyone and everything the rest of the meeting.
The missionary meeting, although full of good intentions, seems to be more about refreshments and gossip than helping the oppressed, even though I am sure the ladies didn't see it that way. The ladies talk for a bit about the Mrunas, then "Immediately thereafter, the ladies adjourned for refreshments." They then spent the rest of the time gossiping about the Robinsons, teasing Scout, and getting into verbal entanglements. I am sure the ladies were interested in the Mrunas, and felt that they were compassionate for knowing about them, but sure didn't do a lot to convert it into action.