It might not be the easiest thing to pick up on, but Myrtle Bouchard—Miss Merle, that is—is actually connected to another plantation near the Marshall's, called Seven Oaks. This means that, like her pals the Marshalls, Miss Merle and her family got rich ruthlessly exploiting slaves who had no choice but to work until they died, as well as poor whites who didn't know any better. That also means that—like her Marshall buds—she's both a symbol of the way things used to be in the South, and a symbol of how broken and useless the order she represents is now.
Sure, she kind of tries to make herself useful, bringing sandwiches to all of the folks gathered at Mathu's place, but take another look at how she acts when she's doing it. Lou Dimes tells us that, as if she really has no idea what she was doing, Miss Merle shows up and "just started dishing out the sandwiches to the first person she got to, and fussing the whole time" (11.15).
At first, that might not seem like it means much. It might look an awful lot like straightforward description. But think about it for just a couple of minutes longer. Lou feels like he has to comment on the way that Miss Merle isn't handing out the sandwiches in any order, which implies that, normally, she'd serve the white men first. Miss Merle is so confused and frightened by the whole situation around Beau's murder that she's forgotten all about keeping up appearances.
Not only that, but her constant fussing about the sandwiches is her way of keeping her mind off of what's going on around her. She may not be drunk on a porch swing or poking around for pecans with a cane, but she's just as cut off from reality as the Major and his wife.