Johnny is really powerful. He's clearly got some kind of happiness magic that just makes life around him more enjoyable. He can just create a party at will, he traveled the world without any money including somehow backpacking to Hawaii, he just so happened to stumble across a secret castle that had been hidden from human all the way into modern times, and instantly makes the 10/10 girl there fall in love with him.
Then there's the scene where he just fucks with light. He shows Mavis how to somehow look directly at the sun, without coming in contact with any sunlight.
It's just like...>"I really wanna see the sun, but I can't be in sunlight">Oh that's easy, just go like this"
And it just works? Magic exists in this setting, and Johnny can do it.