Mandy is an amazing cook, so it's not surprising that we have descriptions of food throughout the book. Sometimes the feasts appear quite scrumptious, but food usually represents more than just itself. For instance, at the funeral reception, Ella notices what a large spread it is: "We stood near the side table, which was loaded with mountains of food, from a whole roast hart with ivy threaded through its antlers to butter cookies as small and lacy as snowflakes" (3.33).
To Ella, this is yet another example of Mandy's awesome cooking, but it's also a symbol of how much Mandy loved Lady Eleanor, and how she's trying, in her own way, to comfort Ella.
To Hattie, however, the food only represents money. She remarks: "Quail eggs are such a delicacy. Ten brass KJs apiece. Olive, there are fifty at least" (3.38). Despite calculating how much everything must've cost, Hattie still stuffs her face, and Olive follows suit.
We're no therapists, but we suspect that the girls are emotionally under-nourished and are making up for it through binge-eating. You might too, if your mother was as awful as Dame Olga.
Lack of food is a symbol, too. On the journey to finishing school, Hattie essentially starves Ella by ordering her not to eat. While this is horrible in and of itself, it also represents how cruel Hattie is emotionally: she withholds not only food from Ella, but also affection and friendship. Since she can't get away with ordering Ella not to eat while at finishing school, she orders Ella to complete little tasks for her, and then emotionally starves Ella by commanding her not to be friends with Areida.
The various magical races also have their preferred cuisines. Elves do light vegetarian food, while ogres seem to prefer their meals still kicking. Centaurs adore apples, leading Char to name the centaur he catches for Ella "Apple." It's notable that Ella shares meals with giants and elves, but must escape her fate as main course for the ogres. If you can peaceably eat with a group, that's a good sign. If they're trying to eat you, that's a bad sign. Culinary anthropology.