Its called "Fantasy of Manners" and/or "Slice of Life", which are 'sub-genres' or classifications of fantasy wherein little happens - the focus being on the characters/relationships rather than anything actually happening.
Unfortunately, there is also very little characterization happening. All of the relationships have almost no progression, and are themselves rather milquetoast. It's realistic (real life is short on serious drama), but this is the less often encountered sort of realism that's in fact bad for fiction. It's simply boring, and one of the things people try to escape by reading entirely made up books.
It is, though, done by choice in JS&MN, of this I have no doubt. The book is just too well-written for Susanna Clarke not seeing it. In the end, it's what she wants to write, which is the best point of view an author can have. Writing should first be for whoever's writing, and only after that for others, that's how we actually get good books. It's also why even very good books will never be for everybody.
