Not necessary, we it body spend a lot of effort to cool and we do a lot of other stuff. If a dragon is sleeping mostly he doesn’t need so much. Also you don’t need any energy to maintain temperature. You just need enough to compensate for what you use. So let’s say they mostly sleep and have really good insulation, they are fine. Or just sleep in a pit of lava.

Cold fusion sounds also feasible, given it’s a mystical creature.