I wasn’t really sold on Mazinkaiser SKL until the titular mech received a gouged eye from a beating. I always found the original Mazinkaiser to be boring, because it was too invincible for its own good–in fact, the Mazinkaiser movie was centered on keeping Kouji away from Mazinkaiser as much as possible, and the final battle itself was rather anticlimactic.
The eye scar was, to my memory, first done in Super Robot Wars Alpha 3, in Touma Kanou‘s path. When Touma receives his new mech, the DaiRaiOh, he gets a rematch with his longtime rival, Baran Doban.
(On a tangential note: Baran Doban’s machine, the Bemi Doban, wields a giant ball-and-chain.)
This scuffle leaves the DaiRaiOh’s right eye damaged. Interestingly enough, Touma refuses to allow the damage to be repaired after the battle, claiming it as a mark of his rivalry. Instead, an eyepatch is installed, which is reflected in the animation for his mech’s final attack–eye scar closeup and all–when it is unveiled against a Balmarian general.
I wouldn’t know if the show’s staff thought of DaiRaiOh when they gave Mazinkaiser SKL an eye scar, but that simple touch made the mech–and the anime itself–a lot more enjoyable for me.