Ceasefire says that all facedown monsters are flipped up (no FLIP effects are activated).

Tsukuyomi, Otohime and cards of the like all say that when they're flipped face-up, an effect occurs.

Technically, since Tsukuyomi is not a FLIP effect monster, would its effect go through Ceasefire, since Tsukuyomi only needs to be flipped up? :confused:

You would be correct. FLIP effect monsters say "FLIP: xxxxxxxxx..." on the card. If a monster says "When this card is flipped face up" that is NOT a "FLIP effect" even though they work the same way in all other regards.