When reading through BoLT, Fui Nienna seemed grim, but not evil. The Nienna in the Silmarillion is of a much more cheerful kind, but I don't believe they are opposite at all.
I would explain the difference as a man at a funeral, compared to a man at a park. At the funeral, the man is grim, as someone has died, but he is still the same person as the one at the park, just feeling different emotions.
All that Tolkien seemed to have changed was the attitude and level of grimness.
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