Yes, I believe in One Piece skin is tougher than bones when it comes to Big Mom and dinosaur/dragon user because that's what the manga tells us.
As you can see from examples I have provided their skin or exterior body is the toughest part of the body.
Nowhere in a manga has it ever talked about their bones or inners being as tough or tougher than their skin. You should look at their skin like armor.
No, I don't believe rooftop Asura Dead Man Game is stronger than G5 Bajrang gun. I do believe cutting attacks are better suited against opponents how are very tough and durable. Zoro had the advantage against Kaido even though Luffy attack contains internal destruction power.