Is this world the first one with high level characters?

[Gluteus_Maximus]

Presumably the wizards/sorcerers that V soul spliced with were from this world's tenure, as any fiends would have had their memories erased and would not know about older worlds' souls. Not to mention, Xykon is less than 2 centuries old, and this world is "A few thousand years old" given that the world is about to become too unstable due to the rifts. If it takes less than 2 centuries to become epic level, then there have definitely been more high level characters from previous worlds.

On the other hand, this world being a "Self-aware stick figure fantasy parody" might mean that some other worlds didn't even have a concept for "high level".

On the subject of xp-fodder monsters, the gods must have created them using the strands of reality before humanoids were conscious (probably means the narrative from SoD that RC provides is false). They aren't a concept that hasn't ever been made before, but in older worlds you can always get your xp from killing other low-level PCs.