Well, let's face it, narrative isn't one of Dragon's Dogma's strongest suits. For the majority of the game you're in a fantastic medieval setting with knights, peasants and priests. You go from humble beginnings to hero of the realm. You fight monsters and evil cults. Etc. Its as generic as it gets. A plot twist SOMEWHERE was sorely needed. The ending was it. At which point you might dispute its quality.
Well, weirdly enough, the ending is both insane and consistent with the game up until that point. Which in my book is a good thing. Let me explain:
The idea that there's some ultimate mystery to everything is well referenced through the game. Your first encounter with Selene is about the Church hoarding secrets about the Dragon. The 'True Nature' of the Dragon is also mentioned as people point out the strange things surrounding Edmun's own vanquishing of 'his' Dragon. Same thing about the 'True Nature' of the arisen and the pawns ever since the Everfall pointed to a mystery behind the rifts, or Selene's and the Dragonforged's 'Bestowals of Spirit'. So its insane, surprising and perhaps exciting but not really out of the blue.
Furthermore, the narrative of the hero who rises from humble beginnings to take on some grand quest but only in the very end goes through some sort of world changing Epiphany is actually classic storytelling. Which brings me to the second point, that the ending is also consistent from a thematic point of view.
The Hero and a handful of love interests are the only people in the game world who won't roll over and die in face of death incarnate. If you ask just almost every NPC in the game, they'll tell you about how futile it is to run away. Not to mention how the big hero Edmun Dragonsbane stands on the defensive, or the spreading of the Salvation death cult.
So the idea that the Hero stands out for his will to live and to survive adversity is a central point of the storyline from the beginning. And what is cool is that you, the Player, also only realizes the true meaning of the whole journey together with the PC. His epiphany is yours as well.
Now onto the ending itself. You ascend to some divine plane where God tells you all the bullshit has been to see if you were badass enough to become God. Well, in here I found some interesting ideas. When you get to choose between living a happy life as a fisher or marching on towards God, you are forced to kill the main characters of the story. The way I see it, its your PC shredding his mortality, his humanity and his history, in order to achieve Godhood.
Personally, the only real disappointment is that the 'true' final boss is too easy