>>34961198As far as I'm aware, Aster wasn't her mentor so much as someone with a special connection to Pokemon that was loved by everyone, including Zinnia. And then when Aster died and the duty of Lorekeeper fell on Zinnia, people felt she was an inadequate replacement and she kind of cracked under the pressure. Though I'm probably mis-remembering some details.
You're right though why was everything but "Aster was someone important to her" not explained in the actual game?