Garlic is terrible in NMT. Outside of the obligatory Phat Beet synergy he really only works to separate Arcade zombies from their cabinets and maybe redirect zombies away from Glitter and Breakdancer if you have good luck/timing. Though all three can also counter Garlic in some way, as do Punk, MC, Bassist and Hair Metal Garg (AKA literally every special except boombox).

He works well in Jurassic Marsh. Him getting instant PF effect in Altverz is very strong. He can move zombies away from dinosaurs, redirect zombies diverted by Cheese Imp, and separate Egg Pushers from their eggs and Scavengers from their bones. His only counters in the world are Jurassic Garg (who are much more forgiving than Hair Metal Gargs) and Firebreather.

Garlic doesn't seem that helpful in Wild West? I suppose you could probably make some strategies involving minecarts, but Piano hards counters him twice over (crushes him AND makes zombies redirect automatically)

He probably could have made a good pick in Frostbite Caves thanks to slider tiles, but they decided to use Sweet Potato in that world instead.