Too late now but it's probably not worth complaining about it. If they couldn't mount a plate properly would you really want them to try and fix it? It's just screws that go into a plastic bumper, not a big deal imo. Besides, it'd take 5 seconds to fix it yourself. It's not gonna drop the resale value anymore.
But it's your car, do what you feel is right. It shouldn't have left the lot like that. Are you gonna try and make them buy you a new bumper cover now or what's the plan now? Maybe see if they'll throw in a front plate mount bracket for you unless you plan to go with the tow hook bracket style?
View my fuel log 2014 Mirage SE wussie cvt edition. 1.2 automatic: 37.7 mpg (US) ... 16.0 km/L ... 6.2 L/100 km ... 45.3 mpg (Imp)