While 'curved' is not as specific as 'en chapeau de gendarme', I don't think you can go far wrong with it, even if all subframes probably have a degree of curvature somewhere. More specifically you might call the shape a normal-distribution curve or a bell curve, I've even seen it called a 'coolie hat curve', but those would hardly work in this context.

It would be nice to know in which dimension the thing is so curved. I imagine it is across-the-beam, but that is by-the-by.

I would prefer 'subframe' to 'cradle' for a range of reasons related to the different possibilities of chassis construction, but primarily to get away from the idea of the engine cradle (a thingee on wheels) used to support and transport an engine separately from the chassis, even if in the context there is undoubtedly no risk of confusion.

"Aussi appelé berceau de moteur, le berceau de voiture est un élément métallique fixé au châssis. Ainsi, il joue un rôle décisif au niveau de la jonction des différentes pièces mécaniques. Généralement, il permet d'assembler entre eux le moteur, les amortisseurs, le réservoir de carburant ou encore le système d'embrayage du véhicule. On retrouve un berceau soit à l'avant soit à l'arrière d'une automobile en fonction de l'emplacement de son moteur."
https://www.vroomly.com/blog/berceau-de-voiture-les-elements...

"Subframes & Engine Cradles
The subframe is the structure below the frame that supports the axle, suspension, and powertrain.
Engine cradles (or k-frames) for internal combustion engines are in the front of the vehicle. For electric vehicles, the cradle wraps around the motor housings to provide mount support and battery protection in frontal crashes. In either case, the subframe performs a critical role in the stability and the ride quality which are central to vehicle dynamics, safety, and in meeting customer expectations."
https://aec.org/page/subframes-engine-cradles

"subframe
A structural component of a vehicle, such as an automobile or aircraft, that uses a discrete, separate structure within a larger body-on-frame or unit body to carry certain components, such as the engine, drivetrain, or suspension."
https://www.definitions.net/definition/subframe