I don't think people are stupid but they are easily swayed on any issue where they have no real opinion of their own.
This is why film critics have a job. They express an opinion on a film you haven't seen so you can decide if you want to see it Similarly all other critics (books, food and so on).
People using TripAdvisor or similar sites look at reviews by other people they have probably never met.
In the case of Europe, I think a lot of people had no real opinion on the EU and that is why they were so impressionable. A hostile press in England played a big part in that.