My radiator is warmer at the top, is that normal ?

It's completely normal for your appliance to be warmer at the top, even in the case of radiant appliances. This phenomenon is due to a natural convection movement around the appliance, which creates a temperature difference between the top and bottom of the appliance. 

Appliances incorporating a surface resistance on the front are less sensitive to this phenomenon and have a more homogeneous surface temperature.