It would seem that since she's Mexican, and the plane is a Mexican carrier, subject to the jurisdiction of Mexico, the child is Mexican.
I'm sure someone will find a way around it.
A Mexican plane is like a Mexican ship: It's a piece of Mexico and Mexican law applies to what goes on inside. The kid's a Mexican.
I don't suppose there is an enormous market for the route from Mexico City to Toronto, but in principle there could be such a route. Would a child delivered by C-Section over Kansas on a flight from Mexico to Canada be American?