If a show contains different creatures like elves, succubi, vampires or ogres from different mythologies and if I don't believe that those creatures are real or they actually exist will I be committing (kufr/shirk) if I watch these shows?
Watching such stuff is definitely bad for the individual; even if one condemns the viewed action, it still could negatively impact one's subconscious mind without feeling, hence negatively affecting you as a person. And most importantly, it's something that doesn't please Allah Subhanu wa Ta'la. So watching those stuff, especially hentai, is definitely haram and forbidden in Islam, as Islam always aims to preserve one's purity and mental clarity, away from filth and sins/shrik. But it is not shirk/kufur since you disagree with such actions. I would suggest you make tawba to Allah The All-Merciful, and Al Hamdulilah that you stopped watching such stuff. I ask Allah to keep you steadfast on your deen.