This question is a bit different than the one here, because the NPC is not a jerk. In fact, they're actually quite nice. However, this is not the first time the GM has done this.
In our game, we have a legendary NPC on our side who just gets more and more likable and perfect the more we play. The NPC, who was a surprise addition, is legendary, ancient, immortal, and a messenger of the gods who is the most likable, friendly, charming, powerful, and humble NPC to ever NPC. The GM constantly emphasizes how great this NPC is, how much they love them, and how much everyone loves them, especially how powerful they are.
The GM has done this sort of thing before, with another NPC who was basically the same thing, except less charming and more brutal. But again, the legendary, unstoppable, utterly broken NPC who was the most powerful NPC to ever be powerful and had to have the final killshot of the game, and made the whole game about them and their destiny.
I don't want to just be a cheerleader to these people, but they're obviously self-inserts, so I don't want to offend. What do I do?