HN, as its name indicates, do not aim at a universal demographic. The demographic it does cater to is also very likely to use a keyboard heavily during the day, and so retains some good habits of longform discourse even when contributing via mobile.
Right so then the bad habits are not tied to mobile but is rather a cousin issue. Most people use mobile and, independently, longform discourse does not have mass appeal.
As I read on a site on with no images or video and entirely in the form of thoughtful responses via mobile device.