The problem with spawning them with helmets is that the helmets break. A workaround could be a way to spawn them with their own skull as a helmet, which I am unsure if your code is capable of doing currently, as there isn't an individual id for each skull. Maybe a new variable could be added for skulls, that allows the entity data to be entered like

"skull": ["minecraft:skull", 2]

and this would spawn the mob with a zombie head in the helmet slot which doesn't break from the sun, and would cause the mob to still look the same. The harder way would be figuring out how to prevent a mob from burning in the sunlight and adding an option to make a mob burn in sunlight or not burn in sunlight.

I am trying to get this to happen of my pack as well, having a dungeon bombarded with zombies (with the help of spawner control) just to see them scatter outside and burn is quite disappointing

I would appreciate it if you share the config script you used to spawn them with hats.. thats still better then nothing

@rooben777
{
'mob": "minecraft:skeleton",
"result": "allow",
"armorhelmet": ["minecraft:cookie", "minecraft:skull:0"],
"canspawnhere": "default"
},
{
'mob": "minecraft:zombie",
"result": "allow",
"armorhelmet": ["minecraft:cookie", "minecraft:skull:2"],
"canspawnhere": "default"
}

not sure if the cookie trick works still ! not sure how to remove the skull pic in the code but there actually supposed to be " Colon( : ) skull followed by another Colon then metadata"