In.
For the record, is Hidden Power okay on other legendaries? Looking mostly at [HP: Electric] Articuno/Suicune here.
And what about ML/LK/Nightmare Jynx?
You may use Hidden Power on other Legendaries because, let's be real, only Zapdos and Raikou are really the problem.
As long as it isn't Perish Song + Mean Look/Spider Web + Sleep Move all on the same set, it's legal. So, yes, that Jynx set is legal. Note that you'll probably want to play on Grotto or some server other than PO main if you do use it, though, because PO main
will boot you to Ubers for using that Jynx set.