Peyton obviously isn't having anything remotely close to what I would call a "good game". But you'd have to be a damned moron to not see that the entire team is playing terrible (and Seattle's defense looks great too). Denver's o-line is struggling (bad snap, false starts, etc), the defense can't get a stop on 3rd down and is making bad plays (Carter's pass interference in the end zone that would have brought up 4th down).
And the play calling on offense hasn't been that varied. Peyton hasn't been great at all...but he's not getting much help either. Seattle is just outclassing them on all levels right now.
Also, I wouldn't have gone for that 4th down at the end of the half. Even 3 points is a moral victory. You can't keep going down the field and coming up empty. I get that Seattle gets the ball to start the 2nd half, but the only way out of this hole is to ask your defense to carry you and keep chipping away. Leaving points on the board isn't going to get it done.