When an animal increases in size, its weight increases by the cube of the increase, while surface area only increases by the square. That means the mutant mosquitoes would be trapped on the ground, helpless. Their legs would not support them, and their wings would not let them fly.
Mosquitoes lay their eggs in murky, watery places, such as swamps, not on dry surfaces, as depicted in the movie.
Around 34 minutes into the movie, as the camera is moving across the campground showing the dead campers, a corpse wearing green socks behind a tree on the left side of the screen can be seen adjusting their legs.