Learn English – the meaning of “duh” in this sentence

In this case, though is being used as an adverb.

As my New Oxford American Dictionary tells it:

though: adv. however (indicating that a factor qualifies or imposes restictions on what was previously said).

So in this case, you can think of it as such:

• Person A: [Picture of dog] _{(thinking: Man this dog is ugly!)}
• Person B: Cute face though _{(thinking: Yeah, he's ugly but/however his face is really cute)}

In other words, he's ugly, but we're going to limit his ugliness to everything but his face.

In your example, since person B just says "that face though", it's left to the reader to infer the cuteness/silliness/ugliness/craziness that is different between the face and the rest of the dog — presumably evident enough from the picture.

'To blow the curve' is a phrase that originates from certain grading systems found in American universities. Usually on any given exam the grade result for the class will fall into a basic bell curve shape. If there are no 'A' grades on the upper end of the bell curve, the professor will shift the curve up until there is at least one 'A', but the spread is still a basic bell curve. So, no one is graded against an absolute standard but against everyone else in the class.

Let's say that the bell curve for an exam has a peak in the mid 70% and the range is 50 to 85%. The professor shifts the curve until 85% is equivalent to 100% and everything else shifts with it. However, if there was just one student who scored above a 95%, then the curve can't be shifted and everyone is stuck with the grade that they made. Which means that a lot of people failed. So the one guy who got the 95% 'blew the curve' and is very unpopular with his classmates for a while!