Nextdoor doesn't make people be shitty human beings, but it is a local forum that augments or replaces the sorts of venues that people used to be shitty in -- town hall meetings, HOA events, neighborhood watch, political rallies, etc.
Arguing with people on the internet is (heh) usually a waste of time. But pushing for decency and respect within your neighborhood is not a waste of time if you value strong communities.