I don't think it's stupid. As some have said, if Mitt was the school bully, combined with the idea that he likes firing people, combined with other stories from adulthood - it could represent a pattern, and a kind of personality that a lot of people aren't comfortable with.
At what age do you start taking people's actions & words into consideration? If the haircutting incident was a one-off, I don't think it's that big of a deal. If it was a pattern of behavior for Mittney in HS, college & adulthood - it's kind of a big deal.
Beyond that, his claim that he doesn't remember isn't credible to me. That is more incriminating than the actual incident, imo.