A platitude is a meaningless conjecture that is put forward as if it is original. I am not going to claim to have found them all, but let's start with the title. "Good Country People" is not only the story's title, but is the major platitude uttered by Mrs. Hopewell--the belief that someone from the country is morally superior to a city person. While it is a ridiculous notion, is used to be and is sometimes still commonly believed. Mrs. Hopewell says they are the "salt of the earth"- another platitude and cliche.
"That's life" is commonly stated by Mrs. Hopewell which of course is a platitude, because everything that happens is life.
In another example, the notion that something "makes the world go around" is meaningless as well.
Mrs. Freeman's "I wouldn't say it was and I wouldn't say it wasn't" could be said to constitute a platitude.
Here are some more flat, dull remarks from the dull Mrs. Hopewell (isn't her name ironically perfect!):
"Everybody is different"
"It takes all kinds to make the world."
"Nothing is perfect."
"...not the kind that you would want to be around you for very long."
"people who looked on the bright side of things would be beautiful even if they were not."
"she didn't have a grain of sense."
"a smile never hurt anyone."
"You said a mouthful." (the Bible salesman utters this one)
"overflow with hospitality"
"we all have work to do"
"His breath was clear and sweet like a child's"
These platitudes demonstrate the torpid mind of Mrs. Hopewell who is deceived by the Bible salesman. They also throw into sharp contrast the grotesque ending to O'Connor's "Good Country People" after the utterance of a platitude by Mrs. Hopewell: "I guess the world would be better off if we all all that simple" in reference to the Bible salesman. Mrs. Freeman then remarks, "Some can't be that simple...."