Are you forgetting the restriction that Zoro doesn't know anything about her ability? He wouldn't be smart enough like Momonga to find a way to block the effects of her mero mero mellow. Besides she was breaking Pacifistas with a single kick. If Zoro did knew about her ability then yes, I would agree with you.I'll say Hancock mid or high diff.
I know she's strong but I don't think she's strong to the point of low diffing Zoro... this may change as we see more of her though.