NextDoor is a bad representation of every neighborhood, because the vast majority of people who live anywhere don't give enough of a shit about their neighborhood to waste their valuable time on a social media platform solely focused on it.
You'll get some decent people, those trying it out, those new to the neighborhood, the people looking for their lost pets, the occasional annoying guy who really, really, really needs people to know about his mulching business. They're all fine.
But the people you will find on there most often are the busy bodies, the HOA assholes, the NIMBYs, and most of all: the retired homeowners (boomers) whose world has become so small and insular that the goings-on of the neighborhood is a primary interest to them. NextDoor is a perfect companion to the binoculars Edith keeps on the end table by the front windows in the living room. The ones she uses to watch what those kids over at the Johnston place are up to in the garage, just in case she needs to "call someone".