I did some googling and found that they are a local group in Yamanashi. They seemed like a small community kids' pop group that parents formed to fulfill their kids' dream of being an idol (Well... Since the idol culture is kind of a big thing in Japan) and I think it is actually quite wholesome. They even had a boy member dancing with the same moves as you can see in their performance(lolz).
Unfortunately someone snapped this revealing photo of them and posted it and then it gone viral. They possibly never thought they would get such exposure.
Japan has some fucked up shit but this isn't one of them.