Maybe, maybe not
The economic algorithm will have more than just this variable.
On the positive side - slightly less fuel, slightly less wear and tear, quicker boarding times possibly more time in the air if timetables are rejigged.
On the negative side - do landing and other airport fees have a per plane component - resulting in higher cost per passenger. more or larger airplanes are required to fly the same number of passengers around leading to higher costs from operating more or bigger aircraft, more airport slots are required so further cost to airline. If no slots are available then bigger aircraft required.
etc etc etc
On the whole I can superficially see why on one level a j seat costs sometimes 4-5 times a whY seat.
Is it true that a J passenger subsides the Y passenger?