Bad parenting (on a bus)

I posted this to Facebook earlier:

"I observed earlier and example of "how not to parent". I was on a bus back to Clifton when a boy of about six or seven started to be told off; he was being a bit mouthy - not rude, just being silly and talking a little bit of nonsense, as kids that age tend to do. Rather than perhaps advise him to keep his voice down a tad, his mother instead loudly asked him if he was stupid, called him stupid and told him to "shut [his] mouth".

I'm not a parent (yet) nor would I want to tell others how to raise their kids but that seemed to me like a completely inappropriate way to handle the situation; I thought it was a gross overreaction towards a child who wasn't being much of a nuisance, if at all."

 What do you think?