I think it seems like that, but here is my take on the reality of it, and this is based on thinking recently about the overall storyline of the three sons of bartleby becoming the leaders of the fire storm and ice titans, and myth being formed from the shadows of fire and ice. I think it seems to me that the bosses are increasingly becoming myth, while a lot of enemies are fire and myth. Meaning I think that as Azteca progresses, there might be more and more ice bosses. The whole idea of the winds carrying folks between zultun dock, twin giants and floating mountains relates to ice, and I think that since the defeat of Malistaire ended with a battle against many fire bosses (or fire titans = dragons), coupled with the next main storyline beginning in Celestia and finishing with what seemed like a larger number of storm bosses in Avalon, maybe Azteca will carry on into Khrysalis and have more and more ice titans (Giants)
You outlook on it all depends one what school you're playing really. Personally I thought there was a lot more Ice, Myth and Storm. They balance the schools pretty evenly though. Really I never noticed fighting very many Fire at all. I played through it on all schools BUT Fire though.