This is a country with virtually no internet, comparatively few computers (per capita) and, as such, minimal infrastructure to nurture and support high-level programmers.
How then, would North Korea be responsible for major hacking when other countries with vast numbers of programmers could be responsible? China, India, Pakistan, Russia and any number of underground American anarchistic groups are vastly more equipped to do so.
Let's say, for a moment, that North Korea DID hack DYN. Where would they get the expertise? Well, who's their neighbour? China. Why does their neighbour tolerate North Korea rather that simply rolling over it? So they can use North Korea as a pawn/puppet to launch clandestine attacks.
I'm not suggesting China did it, I am just suggesting that it is highly unlikely that North Korea did it.