It means that the characters do not appear realistic. "A stereotyped character" is one who behaves in a way that conforms to a cliched 'type' of person. For example, an army general in a film who acts belligerently and continually tries to start wars would be "a stereotype", because this type of character has been used many times in films. Such a figure would not appear to have much depth to his character. Another phrase for this is "a one-dimensional character".