Scars in one piece is more to do with some representation than powerlevels...for example, Luffy has scar under his eye from his childhood because he carved with a small knife....and don't have single visible scars except for Akainu scar from all the damage he took in fighting throughout the story...
Same with Zoro...he has scar from Mihawk eventhough he took damage from other opponents....
Shanks had many duels with Mihawk and yet only visible scar he has was on his eye....