Are you surprised? I'm not surprised. I don't think anyone is surprised. Pokémon is one of the more popular franchises on Kotaku, too, especially in recent days since the release of X and Y.
The full list of "most reblogged" video game topics on Tumblr this year is pretty interesting.
Animal Crossing, also not surprisingly, comes in second. It's followed by Kingdom Hearts, Legend of Zelda and Mass Effect.
It's not so much a gauge of the best games so much as the games with the most storied backgrounds, with the biggest fan bases and a long shelf life. Take Team Fortress 2. That game came out six years ago but people still play the hell out of it. And a game like Portal has so many memes and loveable characters that I don't think it'll ever get bumped off the Tumblr Most Blogged lists over the years.
But the year is really not over yet, so if you feel like tipping the favour over to Transformers: Fall of Cybertron you better get to drawing pictures and coming up with jokes for your Tumblr page.