There's no point in ranking Ambipom at all.
I'll put the reason into simple words: Ambipom is completely irrelevant, nobody uses it, and anyone who does is probably trolling the ladder.
The other tiers have an E-Rank mostly because there's a "crutch" present in the tier such as Munchlax/Dusclops who receives undeserved levels of usage. OU doesn't have any particular Pokemon who receive enough usage in the tier and are complete hindrances to every team.
I'll put the reason into simple words: Ambipom is completely irrelevant, nobody uses it, and anyone who does is probably trolling the ladder.
The other tiers have an E-Rank mostly because there's a "crutch" present in the tier such as Munchlax/Dusclops who receives undeserved levels of usage. OU doesn't have any particular Pokemon who receive enough usage in the tier and are complete hindrances to every team.