They aren't real world mirrors.
Interesting that no one points out that they're connected to Flect's Quirk, which reflects everything that touches him. I have the manga booklet thing that came out as well, and even in the movie you can see Flect fire out the energy he reflects through the "mirrors". Yet no one mentioned him shooting air out of those "mirrors".
"Arachne, reflection-control exoskeleton" From the manga book thing. The exoskeleton is literally tied to his reflect Quirk. No duh they're connected, why would natural mirrors be the same color as Flect's Quirk? Flect's skin is normal, but is blue when his reflect Quirk is activated. The same color as the mirrors.
Nothing stated in universe that they were mirrors in the first place. Mirrors aren't blue, the reflect like and show a reflection of the environment. If they are mirrors, that means they're only reflecting blue light and absorbing all the other light, but they reflect red lasers?
Edit: Quirk Relfect: "It allows the user to reflect all types of energy. He usually wears an exoskeletal device called Arachne that controls reflection, changes the direction of energy, and maintains his balance in the air."
Another points out to the "mirror" that floats underneath his foot when he starts flying. Stating that it makes him float by reversing the impact.
People saw energy weapons being fired and bouncing around on disc shape objects and just jumped to lasers and mirrors. When the objects are never called mirrors, even in the book they are labeled as part of his exoskeleton. The energy weapons are never called lasers, and are dodged by non super human characters.
While Izuku was only hit by them when Flect made their movements hard to follow with his Reflect Quirk. Meaning the attacks weren't that fast.