It is safe to put things on radiator covers, and actually would be encouraged as an attractive place to position accentuating features and decorative items to complement your home radiators and décor in general.
The way radiator covers are designed means they help radiators project heat outwards rather than above, so the top of the covers will keep cool, and offer an ideal location for any decorative objects you might choose.
To find out more, explore our blog, How to decorate above a radiator.