That would depends on a lot of factors, electronics are usually expected to heat up, up to a certain threshold. Since I'm living in a tropical country, the room temperature here is hot so the dock can't dissipate the heat fast enough and it heats up, try to see if you are also running graphic intensive games and is charging at the same time. Try to see if different apps/games affects the screen heating, some apps/games aren't optimized enough and uses more power heating up your phone faster.