A bad spark plug will most likely cause stalling only once it’s really hot so I’d rule that out. It’s cheap though and should be replaced periodically. I’d say about twice yearly.
Kymco 125cc have an anti-pollution system on them and it usually breaks down within 5 years or sooner causing backfires when decelerating. The parts are expensive and mechanics never replace them because the bike will run the same without it.
If your bike backfires when decelerating, I suggest you disable the anti-pollution. Often the kymcos have an access panel in front of the seat where you put your feet. If you open that, you will see a big hose 3/4" diameter. Unplug it and stuff your old spark plug into it, it’s the perfect size and it plugs the hose permanently. Your anti-pollution is disabled and the backfire issue should be solved.
If the bike backfires when accelerating and stalls when you stop quickly, I suspect your CDI could be faulty. Thats about 800nt installed on a kymco.
DO NOT let anyone blow pressured air into your carb in an attempt to clean the air orifices! These carbs have a thin rubber membrane and forced air will ruin your carb. If a mechanic adjusted your carb and the problem remains, I think it’s either the anti-pollution system or a weak spark(CDI).
Many shops will have a spare CDI they can install for a test drive. I would do that first and see how the bike runs.