Oops-- yes there should be some sort of alert in the cockpit for that.

Not to defend a flight attendant who did something that major, but, until very recently, the upper deck doors were ALWAYS armed and so we were not supposed to ever touch them to arm/disarm them as they always stayed the same. Maybe s/he didn't think to arm them because s/he had a flashback to the days not too long ago when they were always armed.