This is a another version of Curing trap phobia, but as you can guess, the party is now using Insight checks on all NPCs they meet!
I've cut down encounters with unnecessary NPCs, so every NPC they interact with is important, or somehow able to give information or a (mini) quest.
These players have read a lot of DnD stories, and I think they are worried about a quest-giver that turns out to be the bad guy, or hiding something. I have used no quest NPCs like this.
How can I cure their NPC phobia?
Note that in the future, I have a quest about the cliche corrupt mayor who asks for something to be taken care of, but turns out it's for his shady business. I would expect answers that do not simply suggest not using "bad NPCs" at all.
I was thinking about using Passive Insight, but I'm concerned with the low Wisdom the characters have (cleric is going STR build), so there will be no chance of them spotting the hidden agenda.