My opinion on Kyurem is probably heavily influenced by how often I saw cutesy fanart of it, particularly when the Black and White were first released. If I recall correctly, I saw a lot of fanart and comments regarding how Kyurem looked rather weak, or even pathetic, particularly compared to Zekrom and Reshiram.
The same's happened with Heatran. I really had no opinion on it at the start. But when the Air Balloon item was released in Gen V, I saw a good set of fanart showing Heatran floating around in a cutesy manner, tied to a little red balloon, and now I can't stop imagining that whenever I see the Pokemon.
I guess once I get a cute image in my head of a particular Pokemon, it's hard to remove. Sure, I can still see Kyurem and Heatran as being badass, but the cute side dominates now, at least for me, haha.