What was the irony in the statement, "You are a fortunate girl. You live in a Christian home with Christians folks in a Christian town"?

Mrs. Merriweather's comment to Scout is ironic because the town of Maycomb and its whites have not behaved in a Christian manner toward Tom Robinson. Even though Atticus showed at his trial that Tom Robinson could not have raped Mayella Ewell, the jury still found Robinson guilty, upholding the racist idea that the word of a white should always be believed over the word of a black.
This belief runs counter to the Biblical idea stated by the apostle Paul in Galatians 3:38:


There is neither Jew nor Gentile, neither slave nor free, nor is there male and female, for you are all one in Christ Jesus.

In other words, a true Christian town would have judged Tom Robinson, a fellow Christian, based on his character rather the color of his skin, seeing him as equally one of them in Christ. It was, as well, not charitable or loving behavior, which Christianity also calls for, on the part of the townspeople to condemn an innocent man.
However, all of this is lost on Mrs. Merriweather, the mouthpiece of white Maycomb orthodoxy. She confuses conventional behavior convenient for middle-class whites with Christianity and is angry at the local blacks for being upset at the trial's verdict, calling her maid's "sulky" behavior unChristian, not the unjust verdict.

Mrs. Merriweather directs this comment toward Scout during Aunt Alexandra's missionary circle. The irony surrounding this statement concerns the fact that Maycomb does not embody Christian ideals. In actuality, the community of Maycomb consists of racist hypocrites, who openly discriminate against African Americans on a daily basis. Mrs. Merriweather hypocritically believes that since the majority of the citizens in Maycomb identify themselves as Christians, it is a better place to live than foreign countries or environments. Truthfully, the community of Maycomb marginalizes its African American population. Tom Robinson and other black citizens suffer racial injustice at the hands of the predominantly white population. Overall, the town of Maycomb does not follow Christian values, and its overt prejudice adversely affects the minorities throughout the community. Despite its Christian label, the community of Maycomb embodies racial injustice, racial inequality, intolerance, prejudice, hypocrisy, and hate.