One of Ma's "Rules of Dining" is to "place plates upside down," and another rule is to set the "glasses bottom side up." Ma sets the plates and glasses like this so as to prevent the dust from coating the eating surfaces of the plates and the insides of the glasses.
The dust storms were so severe in the 1930s that the dust would find its way inside people's homes. If the table was set with the plates and glasses the right way up, then the dust would coat the surfaces of the plates that the food would be served on and the insides of the glasses that people would drink from. By placing the plates and glasses upside down, one could keep the surfaces of the plates and the insides of the glasses nice and clean.
Ma's other "Rules of Dining" are also predicated on the same idea. One of the other rules, for example, is that the "napkins [are] folded over forks, knives, and spoons." The napkins are used to cover the cutlery so as to keep the dust off of the implements that people will be using to eat.
The dust storms at this time were so severe that over one hundred million acres of land were damaged. Much farm land became uncultivatable, meaning that crops could not grow, and many farmers had to leave their farms to try to find work elsewhere. The dust was also responsible for the deaths of many. It is not surprising, therefore, that people like Ma would take care to ensure that as little of that dust as possible made its way into the food they ate.