Unhappy? Nah
It's Miu's usual style. If you have been following her works since her Jr career, you would find out that she is just being herself in this new PB. If you have time, check out Noriko's IV which directed by Miu Nakamura, you would notice the similar "unhappy" element in that video.
This is MiuMiu's unique personality we are talking about. Miu always has her own ideas and she always present it in her works. You certainly can't expect that she would act like the other typical idols who always smiles and cheer up and being cute in front of the camera.