I think there also needs to be considered the axis of niche.
When something is good, not liking it often comes down to a matter of personal taste instead of being indicative of quality.
For example, if something is mainstream and bad, people will hate it. If something is niche and bad, people will hate it. If something is mainstream and good, people will like it. If something is niche and good, a fair deal of people will still think it is bad.
The hard part comes from being able to separate the niche from the bad, and figure if your complaints with something are objective or subjective, i.e., a question of quality or taste.
I may not be a huge fan of Steins;Gate but I'm not going to go rage at people for liking it, because when I stop to break down the elements of the show I can see that it contains some really good things.
On the counter side, I thought that Mononoke was fantastic but I'm not going to have expectations that any other person would enjoy it as well, because the only people it would appeal to would be others who were looking for the exact same thing; a weird show about monsters and the evil that humanity creates for itself due to it's own selfish actions.
Steins;Gate is popular because it is mainstream and good, and Mononoke is unpopular because it is good and niche.
Being popular does not mean something is good, but being unpopular does not mean a thing is necessarily bad, either.