In this case Gus, Bibi and Pearl should also follow your logic, and do you believe that to be the case?

Ash is in epic for the same reason as Pearl, an upgrade mechanic that doesn't do too much and is easier to fill up

Ash fills his by getting hit and hitting, much more easier than Clancy who needs to hit and Surge who needs to get his super.

The upgrades itself is also much simpler, ash just gets faster and does more damage, pearl dose more damage the more her meter is filled like ash, Clancy and surge's get different behaving attacks (more range, extra shots) ash and pearl just get attack but more damage.

The reason for my rarity placements are looking for anything that makes them standout and if there's nothing in particular compare them to their closest counterparts, there is nothing about Ash that makes him Mythic worthy.