>>31798758While Cynthia was just a boring, she had a role. While her role was also bad, in my opinion, it got one thing across. The way she gave you shit throughout the game, appeared on Mount Coronet, went to the Distortion World. All of it got across one thing. She's supposed to be strong and experienced. That's it. The fact that she's the Champion was no surprise, but when you fight her, she's strong. The anime also only fleshes out that aspect of her.
This is a case similar to Red's. He has no dialogue, and no clear personality, but he's loved by many because of how strong he's supposed to be. The same applies to Cynthia. While Red had a game of his own to boost his popularity, Cynthia had her looks. Her simple role as being someone strong alone makes her better than Diantha, who has almost nothing going for her. Diantha has a good team, but XY in general were easy and there was never any chance of losing to her.