So why is nearly everybody's perception of Romo that he chokes in the clutch?
Because everyone's first impression of him was him mishandling a field goal snap to cost his team a playoff game. Then the Cowboys as a team underachieved late in the season a few years in a row. The narrative basically became Romo wasn't clutch and couldn't carry a team.
Now whenever you see him throw a costly INT it gets all the focus. It probably doesn't happen any more or less than with most QBs, but it gets way more media and fan attention. Then whenever he does do something good it doesn't get nearly as much attention.