Asserting his position as the dominant male, John does not treat his wife as an equal. He treats her more like a child. He suppresses her writing, one of the ways she expresses herself. In this case, he literally silences her voice, saying she should not work and not even think about her condition. It is likely that she suffers from post-partum depression but it is also likely that John's condescension and manipulations exacerbate this condition. In fact, he does not take her "nervousness" seriously. This is why he laughs at her imagination and "fancies."
John laughs at his wife for using her imagination. Feeling the romanticism of living in a country house in summer, his wife (the narrator) considers that the house might be haunted. John laughs at this because he doesn't concern himself with impractical things. One might also consider this a nervous laugh since John is afraid of superstition.
He has no patience with faith, an intense horror of superstition, and he scoffs openly at any talk of things not to be felt and seen and put down in figures.
John laughs at his wife's obsession with the wallpaper. Rather than listen to his wife's complaints about the walls of her functional prison, he says that the house is making her better. He believes that he knows what's best for her; despite what she says about herself. In each case, John laughs at what he doesn't understand. He doesn't understand that the wallpaper is a symbolic prison just as he doesn't, or refuses to, indulge in creative musings about the country house.