The mediator is an individual who makes or tries to bring two people in worldly matters or things. The medium, on the other hand, is a person who is supposed to be endowed with paranormal, superhuman powers, that it can act between the world visible, empirical and terrain, and the otherworldly or more there, as is that of the dead and spirits.
The answer to that question is that they are not the same or similar. On the one hand, a mediator is the person who tries to mediate between two parties with different points of view or discrepancies. And on the other hand, the medium is the person who has contact with the spirits, which sees the dead, that you talk about, it reads the mind and various paranormal powers.