Is it OK to charge the Mavic Controller with a fast charger?
The Mavic charger has a 2 amp output and my smart charger has a 3.4 amp output.

I believe it is OK , as long as the charger is for a Lipo Battery
Fast Charging any Battery will create a lot of heat, it is the enemy
heat will cause shorter battery life in the long run  
Slower is better --- You don't want to use the battery right after it is used or fully charged
let it cool down, it will last a lot longer

It should be OK, the fast charger will only provide the full 3.4Amps blow when connected to a fast charge compatible device.

Since the Mavic's remote is not fast charge compatible, the internal circuitry of the charger will provide a non fast charging current most probably 2.0Amps.

Check the back or side of the charger and it should give you 2 different output readings for the current, the highest is for the fast charge devices and the lowest is for the non fast charge.

I am pretty sure that this is the way it works for all fast charging transformers, but i would not be 100% of any cheap-O version, with any well built charger you should be fine.

Cheers!

AACY is correct.  A fast charger will not provide the full amperage unless connected to a fast charge compatible device.