I think it takes a special person to work retail. That being said usually someone in management in retail has seen it all.
They are use to people coming in and out and some make far more mistakes than her. That being said unless she was directly addressed I would assume she/he was saying stupid about the cash register or what caused the issue which maybe a glitch in the system she has no control over.
I would either ask how to fix what was done if it happens in the future or just let it go. If she is called something I would bring it up most definitely but if it was one interaction I would let it go. More than likely in retail it could be a customer or she/he is calling themselves stupid when something can't be fixed. They are use to being called and would rather be called that is why they are there in retail. Good luck!