Assuming you don't have the radiator at materials-melting temperatures, and the tyres aren't actually resting on the surface of it etc., I'd say it's absolutely fine
That, but I’d fit a shelf above the rad as any dust that is drawn up by the heat will get baked on. If you’ve ever hung a picture above a rad you’ll know what I mean.
Assuming you don't have the radiator at materials-melting temperatures, and the tyres aren't actually resting on the surface of it etc., I'd say it's absolutely fine