Out of curiosity I have been searching for a 1.2 liter naturally aspirated petrol engined vehicle that is more fuel efficient than the Mirage and can't find any. Without brake specific fuel consumption charts It can't be concluded how efficient the engine is per se, but the results below speak for themselves and speak highly of Mitsubishi's engineering.
Per UK test cycle (numbers are 1.2 liter engine and most efficient transmission variant):
Mirage = 4.3 L/100km (65.7 mpg) UK Gallons
Suzuki Swift = 4.3 L/100km (Dual Jet engine, i.e, complex cooled EGR and dual direct injection).
Hyundai i20 = 4.8 L/100km
Volkswagen Polo = 4.7L/100km
Nissan Micra = 56.5 mpg (UK gallons) *Nissan offers a supercharged engine variant that is more efficient.
If anyone has additional info post below and I'll add it. If the test cycle is different or our of the UK please provide the comparison vehicle's data as well as the Mirage's data (in that test cycle) so we can at least get a relative comparison.