In "The Importance of Being Earnest," food might fulfill several different roles. On the one hand, food seems to be present in most situations with conflict, such as the women's insults over tea and the men's fight over muffins.
Moreover, food seems to represent the basic desires of people. Therefore, food also represents gratification, satisfaction and pleasure. In a novel that exposes the vices and follies of society, it makes sense that food is constantly present as a vital component of this quest for pleasure.
Finally, food also represents frustration. As pleasure in other areas of life cannot always be achieved, the characters often indulge in food to compensate for the lack of satisfaction in other areas of their lives.