It's not so much that good is dumb so much that good has no reason to suspect that Evil is sitting at the breakfast table plotting against them.
The Jedi became alienated because they believed that their work was done: The very early days of the Republic, it was ruled by the Jedi but eventually, they stepped back to become advisors and peacekeepers.
Then centuries ago they thought the Sith were all wiped out.
It becomes very easy to just... sit back and let the Republic take care of itself, only coming out when the Chancellor wants your wisdom or here's an incident that's escalated to the point that violence is imminent, and just focus on self-improvement and training the next generation of monks.
When everything's perfect, you need to find ways to be productive. Without regular mental stimulation, you start to deteriorate. Complacency is one of the sources of anomie, and generations of anomie lead to the breakdown of culture and society.
In the case of the jedi, this breakdown manifested in them being too comfortable in their ivory tower, becoming more insular and dogmatic and then they get taken by complete surprise and are willing to engage in moral compromise to solve a problem they never saw coming.