Our experience has been the complete opposite. Forms are almost never on the plane. I always fill it out ahead of time.Well, OK, but I have been to Cozumel many times and I do not remember any time when the forms were not passed out on the plane. I'm not saying that it never happens, but it has never happened to me.
Actually, it's three days ahead of your arrival date.https://siat.sat.gob.mx/PTSC/DPE/ You can only fill them out 2 days before departure. Port of entry is Cancun and check point is Cozumel. Very easy