Sagan had great results as a junior in mountain biking, cyclecross and road, he had great results when he first became a pro and is now having great results in his first tour. He's not come from nowhere to deliver unexpected victories, he is just living up to the promise and expectations that alot of people have had for him over the last 2 years or so.
It is a sad situation that people suspect doping immediately not based on any evidence or even innuendo but on him looking fresh. When putting in big efforts (in whatever sport) some people look like that, some grimmace and contort their faces...it isnt a good signifier of how hard they are pushing it.
Apparently he is a really quiet, polite and unassuming person away from the bike
Not sure if you saw it live or not but on the live footage it showed him getting towed most of the way back by the Katusha team car.