She gets trashed in these polls, because she acts as the bad guy against the carnations, wants to manipulate/corrupt Hana and so on. Since these acts put her into direct opposition or at least represent competition for the player character, she gets left in the dust regarding popularity. Her age may play a factor, but only a minor one.I mean, she gets trashed in the character polls for a reason, but hey... can't buy sophisticated taste.
Speaking as a player, the grandma thing is just one of the pluses.
However, as others already mentioned she brings something unique to the table, playing mind games with the protagonist with regards to his moral compass, while at the same time setting the stage for the battle of carnations, the main theme of the story. Therefore she is not only a very intriguing character but also an important one, because without her, pale carnations would be a different, probably much less interesting, game.