I get what you mean, Oda is inconsistent as hell so a lot of the time we tend to scale based on perceptions rather than feats.
However, I think we must give feats some weight too and I don't think the series ends with Aramaki - maybe even Fujitora - having better feats than Garp.
Regarding Big Mom, I accepted her as a bad clown and Oda's worst mistake and moved on.