Does Hallar, the Firefletcher's ability trigger each time a multikicker spell is kicked? Or just once for a kicked spell?
The ability happens once.
Hallar, the Firefletcher's ability trigger whenever you cast a spell, checking if the spell was kicked. Since multikicked spells are still a single spells, there is only one copy of the ability.
It triggers once. While the "if it was kicked" clause limits when the ability triggers, the triggering condition is "a spell is cast".
Kicked spells —even those kicked multiple times— are still only cast once, so the ability will only trigger once.
Kicking or multikicking a spell simply affects the cost to cast that spell.
702.32a [...] “Kicker [cost]” means “You may pay an additional [cost] as you cast this spell.” [...]
702.32c [...] “Multikicker [cost]” means “You may pay an additional [cost] any number of times as you cast this spell.” [...]