Hi Hwan Lee "Unpopular opinion" and "not a popular opinion" both suggest an opinion that is disliked, but you may be able to use "not a popular opinion" when it is an opinion that is in the minority, but not necessarily disliked. Hope this was helpful! Julia

Yes, it means only a small amount of people have this opinion. It also implies that most people would dislike this opinion a lot rather than just disagree - someone with an unpopular opinion may not bring it up in conversation as other people could get angry.