Well, he is remembered for being schizofrenic and violent. He became king at the age of 17, and it was clear that he wasn't sane. He married his first cousin Caroline Mathilde, but took a prostitute as his mistress. They would tour Copenhagen and trash everything they could get their hands on.
After that he met a doctor, Struensee who helped him calm down a bit. Struensee started an affair with the queen, and he became the ruler of Denmark for a short period, until Christians step mother, dowager queen Juliane Marie staged a coup. She had Struensee executed and the king divorce Caroline Mathilde who was arrested aswell. When his son crown prince frederik (VI) became of age, he staged another coup and ruled as regent until Chrisitan VII died in 1808.
He is not the least popular king, just a tragic figure. One of the least popular kings is Christian VI.