While I haven't seen Brendan and the Secret of Kells, I have to say that the other two movies don't really hold a candle to Ponyo. Even Up, which is far and away the best of the two was rather uneven and could quickly go from sublime to standard American animation. I'd say that Ponyo, while not up to the level of Mononoke or Totoro, still provides a deeper, better animated experience with little or no low points.
Besides, these are the Oscars not the Let's-See-If-Miyazaki-Outdoes-Himself-This-Time awards.