The disappointment experienced by the characters is mainly the lack of money versus the fantasized version they have of what happiness and contentment should be. Madame Bovary had come from a well-to-do family when she married Dr. Bovary. Both were at first content with what they had.
Yet, Flaubert lets us in the know of Emme's tendencies when she was younger in the convent, how she lived on romance novels and romantic stories where people always dressed well, and nice aromas always filtered, and she stayed in that fantasy. Hence, the preoccupation she has with money and riches occupied her mind and put values and common sense in second place. She felt of her current status as dull, boring, intolerable, and classless and kept day dreaming about a life of richness.
Later in the story, when the Bovaries receive the invitation to the Marquess home and she dances with the Viscount, she sees THAT as the perfect moment of her life. Nothing is important: Not health, nor family- just the fantasy.
Therefore, Flaubert may show dissappointment through his characters in that all that was material and superficial was what was important to Emma, and that there is a massive chasm between her comfortable life and that she aspires to live in opulence and ritual.
One way in which Flaubert criticizes the middle class is through the constant manner of dreams that seems to be evident. Emma is a protagonist who believes in the authenticity of her own dreams and this is what motivates her, galvanizing her into action. These dreams are caused by the banal condition of her middle class nature. While they are present, they seek to remove Emma from her reality and not allow her to properly function within it. It is these dreams that are the cause of her undoing. Emma lives with a condition where constant melancholy results due to the wide chasm between her dreams and the conditions of her reality. Part of this is caused by her middle class predicament, for Charles suffers from the same condition as the novel progresses. Yet, this might be an overall statement about the nature of subjectivity and how dreams can only be effective if they help transform what is there and not seek to fully transcend it.