These statistics here are collected by the semi-government run German technical control organization. Every car in Germany has to be checked by them every ~two years to be allowed in traffic.
This is a different data source: they only check if the care is safe, i.e. cosmetic issues won't show up in the statistic. They also won't see cars that are actually broken. Also consider the skew by owners who care more about their car.
But overall, it might still be fair to extrapolate from fundamental safety issues to overall car quality. The numbers are quite different from what Your Mechanic reports.