My cousin swore by the mirror method and still ended up with bangs that looked like a staircase. She was using kitchen scissors on dry hair, which probably didn't help. We just called it her "experimental phase" until it grew out.
But if using a mirror isn't the main issue, what do you think went wrong with her technique? Like, was she cutting them wet or dry, or maybe pulling them straight down instead of in sections? That usually makes a huge difference.