This is the belief that popularity directly correlates to quality.
It's rare that you get to watch a TV show or film without hearing popular opinion on it beforehand. This often sets up expectations for the soon-to-be-viewed work; the more uniform and enthusiastic praise garnered by people of it, the more likely you're going to anticipate a knock-your-socks-off feature and wait with bated breath for your turn to extol it. After all, forty reviewers can't all be wrong! This is frequently used in Real Life advertising, with slogans like: "Because X (number of) people can't be wrong!".
But quality is subjective, and even then some people look for quality in different aspects. In entertainment, some people value the story in every work, other people value the story in some works but the action in other works. Some people might overlook flaws due to a work having sufficient amounts of relatability, Wish Fulfillment, or Fanservice (sexual or otherwise). And since individuals will differ in tastes, even hugely popular works won't be liked by everyone.
Sometimes the popular vote turns out to be right. Other times, the work might not live up to the hype. Still, the disparity between pre-judgment bias and individual opinion can be jarring and leave an even deeper sense of disappointment than with no hype to set your expectations so high in the first place.
The Latin name for this is argumentum ad populum, or the ad populum fallacy, but the opposite is also a fallacy. A work isn't lesser quality just because it's popular, or lesser quality because it's unpopular, or vice versa for either of those. See Appeal to Popularity for more info.
Some studios take this to mean Lowest Common Denominator instead of universal (if not unanimous) appeal.
Compare Critical Dissonance