I think, he’s promoting a point of view which jumps out because it’s out of the American mainstream, whereas books which push more US-mainstream values like “normalcy and happiness is one man and one woman getting together” or “social justice is achievable by individual effort” don’t appear didactic because those values are so ingrained into that mainstream that they become invisible as didacticism, no matter how hard the authors push them.*

I think you're probably right here. Miéville's lack of mercy towards his characters is one of the things I admire about him as a writer. That said, particularly in Perdido Street Station, I do remember there being a lot of sociological cant that sat oddly amongst the rest of the story--much like "machicolation"--which I think could have been pruned without detracting from the larger points. Though maybe that, and "machicolation", and the characters in TC&tC lacking traits are all intentional? I kind of actually suspect so, given the intentionality of everything else Miéville does in his books.