How popular is Manic the Hedgehog

Not really. He was a main character of a spin-off non-canon Sonic series called Sonic Underground. The show attempted to appeal to the rebellious youth of the 90s with a rock and roll group consisting of Sonic, his brother Manic, and his sister Sonia. They battle the forces of Robotnik and his two henchmen to fulfill the prophecy and find their mother. Inevitably the show tanked, but there are still a few who like it.
He was popular for a while, but his fanbase is very small now a days.