Creators already have an impersonator problem; that has nothing to do with hive. They solve this by linking/mentioning their official accounts in their content, and this is the best solution both for them and for the internet in general. It's simple, low-tech and has few problems. Cultivating /educating this as the thing to do rather than websites babysitting people will actually solve the problem webwide, savvier users will be less likely to be tricked. Giving users the expectation that sites are here to take care of you will make things easier for scammers/impersonators.

That having been said I think a keybase-style verifier bot would be quite good if done in that way, keybase just gives links back to other accounts it's verified in the user profile, and does not elevate or "blue check" the user above other users in any way.

Perhaps this is a different issue, but by the way:
Entertaining notions of "if people don't step up and do X, the blockchain will fail" is an indication of an already failed experiment. If your blockchain relies on users to take voluntary generous action and the overhead and incentive to take that action aren't enough to get it to happen naturally such that you have to encourage it, the system is broken & time to abandon. I hardly think not enough downvoting is a system critical issue though