Is it possible to nerf a mobs (ghast) spawn rate?

[broken canopy]

with datapacks

[spark viper]

Yes, you can modify the biome files for the biomes they spawn in and either configure their spawn weight, or apply/increase their spawn cost

[pulsar edge]

Modifying biome files does make your pack incompatable with any other pack that modifies these. Another solution would be to use commands to despawn (teleport to the void and kill) a random amount of mobs. But that depends if you'd want this compatability <-> performance trade off

[teal birch]

You can for instance use a predicate to randomly kill a ghast or tag it so you don't try to randomly kill it again next tick.

I am not sure if a predicate or a random command would be better for performance here.