Ref: https://discuss.flarum.org/d/25694-admin-dashboard/20
I see now, sorry for misunderstanding earlier. This is probably not possible right now, either flarum/nicknames adds this feature or a third-party extension does (although I haven't looked into the extension in more detail yet).
For the time being, you could just let your members request the change via a discussion, or just assign them permission to change their nicknames and then clear bad ones, it depends on your use case.
