Some of the nominees are competent -- e.g. Mad Dog Mattis -- and some clearly not (Ben Carson for HUD seems like Trump finding a black guy to fill what has often in the past been a black position). About some of them it's too early to say. And Rex Tillerson is way too close with Russia to be trusted. But all of them are more competent than the Tweeter in Chief. It's possible, even likely, that Trump will delegate almost all executive responsibilities to his cabinet, which would be a good thing. Granted, it would be better to have an actual chief executive in charge, but the idea of Trump actually directing policy is frightening.
Trump may well stick to tweeting and corruptly using his office to enrich himself and his family. Which is probably the best possible outcome; furthermore, it's entirely plausible the corruption will stink too much even for the GOP, and he'll be impeached.
And there's also the possibility, given his lifestyle and fatness, of a major heart attack.