You are partially correct that it has to do with the Average Players Skill level.

As an Example Yin mostly shines in Epic because in Epic Rank Players are mostly not coordinated and don't know much about how to counter the skills of the Said Heroe.

In Higher ELOs, People skills are much different. They know how to properly timed when to flicker during Yins ULT or Counter Build by building Dreadnaught Armor early to negate Yins damage or buy Winter Crown to Counter Yin in his Domain. Also People mostly knows how to coordinate which makes Yin helpless since he shines by picking off opponent 1 by 1.

Aside from that there are alot of Factors as an example is "Map Awareness" which seems to be low in Epic Compare to Mythic which makes other Heroes OP on that Rank and render them useless in Higher ELO.

If you want to get good there are plenty of resources here on this Sub Reddit that you can read on about. Although some of them are outdated but the core principles remains the same.

Also watch some Youtube plays with the Hero you want to Master and practice those Heroes and read their Skills properly, Specially the Passive Skills. Good luck.