Clara's defensiveness suggests that she's aware on some level that the Murdstones are treating her and David poorly. With that said, there's an element of truth to her excuse-making: by the standards of the day, Clara is correct when she says she ought to be "submissive," though for a modern reader, the way in which Murdstone exploits Clara's submissiveness is likely to be an argument against it. Furthermore, Peggotty seems to share Clara's assessment of herself as childish and incompetent, since she provokes the whole argument to humor her mistress.