I think it's pretty aggressive to judge someone's intelligence based on how they coached for 2ish seasons. Cronin didn't work for a number of reasons, but he seems generally nice, and there's no reason to believe he isn't smart.
To me, the disconnect between veterans and youth was the main ongoing issue. It has been referenced by former players, media, and Verbeek. I think Cronin thrived in youth hockey where the players were similar in age, goals, and needs. His message applied to most players, and the process was simpler. In the NHL, what works for the young players doesn't work for the vets. They don't need lectures about basics regularly because they are vets. That will rub people the wrong way over time.
Add on top of that the structure and style of play wasn't working because he's never been a NHL head coach before and clearly struggled to adapt quick enough in the NHL. His supporting coaching staff was either ineffective or inexperienced to help him overcome these challenges. In hindsight, hiring a veteran or former NHL head coach to be an assistant on the bench could have helped him learn faster.
Regardless, this guy just lost his job. Maybe we don't need to insult his intelligence because we disagree with lineup choices or PP usage.