"If a machine gun mounted on a light vehicle of that kind could do effective damage to a helicopter, I think everyone would be actively shooting at helicopters."
LMGs/HMGs' role should be to fight infantry, not aircrafts. Tanks should be countered by aircrafts. To fight air, there's other guns, pods and vehicles, like the flak pod or the dedicated flak gun. If a ground vehicle is to decently fight air, it should get a special gun for that job, not have every ground vehicle gunner be able to competently deal with any air threat.
The core issue is that infantry should be able to decently fight aircrafts themselves, and easily overwhelm and kill them if there's enough engineers, as is the case with ground vehicles. For air, this can't happen consistently and the only decent options have both a high skill floor (this is a casual game) and are heavily luck based (I sure hope my lizzle/dumbfire launcher/... doesn't dust!!!). Flak jeeps were a bandaid fix to the nightbird being too fast for people to lock on to with stingers. They should not be a thing.
The infantry situation can be helped with a buff to the NTW-50 to deal actually usable damage against the 3 non-transport helicopters and jets (or something else in that vein), and letting engineers use an HP-only version of it as a gadget, or fixed by adding a lower damage passive radar launcher (can use a portal launcher, just give it passive radar mechanics) and spending the time to balance things with its existence, decently reducing flare duration, doing some other flare tweaks like returning to the old cooldown time, and emphasizing the FXM's midranged-ness, like e.g. making it lock on a bit slower. But that kind of changes are probably too experimental and would require... A CTE.