From dictionary.com
personwho,intheopinionofothers,hasspecial achievements, abilities, or personalqualitiesandis regardedas arolemodelorideal
Coming 3rd or 4th out of 200+ countries in the world (yes I do believe almost every country plays football, though we are not certain if N Korea currently has a national team, though I think they used to), having the least experienced team out of the 32 in the Finals is a special achievement.
IMO in a world of negativity that is 2018, their real special achievement has been making people smile. Even in defeat, they made people smile, and made people proud. They have been role models.
IMO lots of people can be heroes from all different walks of life, it doesn't have to be a 'my hero is more worthy than your hero' thing, nor do we have a finite number of heroes.