The music is too slow for what's going to be happening on screen. Godzilla was big, lumbering, and most of all, SLOW. Hence, the music follows his style. Big, slow, and deliberate.
That being said, I dislike most of what Ramin Djawadi has come up with for Pacific Rim. He did Iron Man and Pacific Rim just sounds like a slightly "bigger" Iron Man. Which is to say that it sounds like a "B-movie" soundtrack and that's not good. Now, to be fair, what he did fit Iron Man, but that in no way means it fits Pacific Rim. And I'm really disappointed at that.
I'm already thinking a better choice would've been Steve Jablonsky (just to slap Michael Bay some more) because he knows how to do big and epic soundtracks. Most of all, he knows how to do tense and exciting robot fight scene music.