Source?
A minivan has a higher center of gravity so in an example where a minivan is cornering aggressively, say to avoid a collision, the rollover likelihood is higher than that of a sports car. As I understand it safety ratings are generally after occupancy protection and aren't concerned with the class of vehicle which goes against what you've claimed.
From the NHTSA website [0]:
"Can I compare vehicles from different classes?
Side crash rating results can be compared across all classes because all vehicles are hit with the same force by the same moving barrier or pole.
Rollover ratings can also be compared across all classes. Frontal crash rating results can only be compared to other vehicles in the same class and whose weight is plus or minus 250 pounds of the vehicle being rated. This is because a frontal crash rating into a fixed barrier represents a crash between two vehicles of the same weight."
And again they are, generally, looking for impact to occupant, for example for the "Side Pole Crash Test Scenario" under "Test Details" they are looking at "Evaluation of injury to the head, chest, lower spine, abdomen, pelvis".
Just because a sports car is a different class of vehicle does not make it inherently less safe. The driver input can change that for any class of vehicle.
[0] https://www.nhtsa.gov/ratings