What does good taste is not a democracy mean?

What it meant in the context of the film (Detention, 2011) is that a variety of people will have a variety of different opinions. Some people might like X, while others don't. Just because a majority of people don't like X, doesn't mean it's in good taste to dislike X. Good taste is, thus, not determined by what's popular, but is itself an intrinsic value.