Hi,
I am a dementia champion and fly in my spare time. My understanding of commercial aircraft and medical diagnosis is very limited, but may help you understand. The stress of air travel itself wouldn't help your mom's condition, but shouldn't do it any long term harm.
The cabin of an aircraft is pressurised to help with comfort and breathing. The higher you go, the less oxygen, although pressurised in the cabin, it will be the equivalent of standing on a tall mountain, so oxygen will be in shorter supply, though unnoticeable to healthy people. This may have triggered very mild hypoxia for your mom, which is a starvation of oxygen to the brain, making us more forgetful and reducing hand eye coordination and lowering the bodies ability to think and process either logic or movement. This would translate to your mom appearing as if her symptons were worse, but it would not be be long lasting, though it may take a short while to return to "normal". As you are aware, Alzheimer's is a disease of the brain that will worsen over time, but may also make the brain more sensitive to oxygen levels in the blood, compounding the effect of a pressured cabin. I hope your mom gets to enjoy many more holidays and wish you all strength with supporting her &I hope this helps explain a little bit.