Don't want to write this by hand? GodCraft sets minecraft:friction_modifier for you with a switch.Get GodCraftOverview
minecraft:friction_modifier is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Changes how much the ground slows the entity down. Lower values make it slide more, like on ice.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:friction_modifier": {}
}