Well, the problem here wasn't purely the negativity. If it was just someone saying "I am not a fan of this song" I would have upvoted them for being honest. What this person said that was a problem was that they liked the song, even though they thought Hanamaru was worst girl. What did the negativity add to that comment? Very little to nothing was added to the conversation by being negative in this instance.

Next time, they could say something along the lines of "I am not typically a fan of how they use Hanamaru's personality in the anime, but her voice here sounds wonderful and fits the theme much better." Doesn't that sound like a much nicer comment? It provides a positive and a negative, while being thoughtful and adding to the conversation.