There are three reasons not to put things on top of a fridge.

First , the fridge is higher than a table or a case,  so the top of fridge is not easy to clean. Put things on top of a fridge, it is easy to accumulate dust and forget to use or eat things on it.

Second, the fridge placed on the magnetic things, easy to cause fridge failure. For example, a lot of families put the microwave oven on the top of the fridge, the vibration and magnetism of the microwave ovens shorten the life of fridge.

Third, some refrigerant in the fridge is flammable gas, if just placed high temperature things, easy to cause the fridge to catch fire.