I wouldn't get worried about a message that it's okay to treat someone so badly and just sit in the kitchen sink writing.
I don't think DS is that didactic.
My point is that, even though the novel is a sort of light-hearted, nostalgic fantasy rewrite of Jane Austen, with two impoverished, marriageable sisters with embarrassing/inept parent figures, looking to make their way in the world, it's pretty reactionary in terms of the rigidity of social class.
Not just in terms of Stephen's labour being taken for granted, but the terms on which the impoverished but genteel Mortmains are on visiting terms with the inheritors of Scoatney Hall while a girl Cassandra knows from the village is at the first dinner but as extra staff rather than a guest. The village schoolmistress (whose non-U speech is indicated) and the solicitor's clerk are condescended to by Cassandra throughout. Ditto the way that Ivy from Four Stones Farm is seen as an appropriate girlfriend for Stephen, and the way in which she is described as good-looking but in a definitely lower-class way.
I'm not suggesting Dodie Smith was consciously doing this, any more than Enid Blyton was when she divided working class characters into good (deferential providers of cream teas, colourful semi-feral children who act as guides, 'simple' shepherds) and bad (criminals, foreigners, local bobbies who don't defer to the commanding Julian), but it's there and uninterrogated all the same under the delightful fantasy.
Whereas Noel Streatfeild's The Circus is Coming (which is one of my favourites, despite being a rather grim novel in lots of ways) completely deconstructs class hierarchies. The initially snobbish children grow up believing they are impoverished but upper-class, and discover when they go to live with the circus that the aunt who brought them up like that was a former lady's maid to an aristocratic family, and they are in fact descended from that family's cook and groom, rather than cousins of the family itself.