If Thane attacks a ship with an already-exposed Fuel Leak, and flips a crits up using his pilot ability, does the flipped crit trigger the Fuel Leak extra damage?

No. Fuel Leak triggers when you suffer critical damage. Having a card exposed isn't suffering damage.

Thanks that’s what I thought.

Cards arent damage, theyre the result of damage.

Possibly worth noting that there's a combination of crits that would trigger it - if they have Fuel Leak and a Hull Breach, and you flip a Direct Hit, the extra damage from DH becomes a crit from HB, and triggers FL.