When Luanne Fayette was trying to make out with Bubba, he had lipstick all over his face and saw Luanne pulling out of the driveway... But Mama was acting oblivious to the whole thing. As intelligent as Mama was surely she could have put that together. Do you think she was just trying to put a guilt trip on Bubba?

No, she honestly didn't know. You can tell when Mama is laying a guilt trip on someone; she really "puts on a show" (see "Bedtime for Bubba" as an example). That's part of what makes the ending of the Luanne episode so sweet, that Bubba is touched by his grandmother's honest belief in him.