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Holy fuck why does every Arceus discussion come from uniformed faggots
OP, a creator deity isn't necessarily all-powerful or even powerful at all, in fact many myths have their creator deities maimed or defeated by later gods or fading into obscurity. See Tiamat, Izanami no Mikoto and Pangu as examples of the former, Brahma as the latter.
It makes perfect sense for, say, Mewtwo to be the strongest, as opposed to a demiurge who is most likely crippled from the act of creation or has little destructive capability otherwise.