The problem here is whether the heat from the electric blanket will be reduced by the protective layer above, if it is then you might want to place the electric blanket on top, but of course then the electric blanket itself isn't protected from moisture, perspiration and other bodily fluids but modern electric blankets are a lot safer than they used to b, but even so do you really want to be lying on top of an electrical device that is connected to your electricity supply, that might become wet?
The answer here is to choose a mattress protector that doesn't affect the heat that passes through it, or at least not sufficiently that you would notice. Anything thick or quilted will seriously affect the amount of heat coming through, so you need something relatively thin. Of course it also needs to be waterproof to protect the electric blanket.
As long as you choose the right waterproof mattress protector then the answer is no, but you need to understand the difference between the right and the wrong waterproof mattress protectors. Again a waterproof mattress protector that is also quilted will reduce the heat coming through. You need a protector that has a thin breathable waterproof membrane, so stay away from anything that uses PVC because it is too thick and is not breathable.
Look at the washing temperatures and whether it can be tumble dried as this gives a clue to how it handles heat. If it can't be tumble dried then it shouldn't be used with an electric blanket. If it can only be washed at 30c or 40c then it might not be able to handle the heat produced by an electric blanket.
We manufacture two protectors that we are happy to recomend for use with electric blankets. They can both be used over the top of the electric blanket without reducing the heat that comes through. They can both be tumble dried and machined washed at temperatures up to 90c so are not affected by the heat produced.
They both have the same microfine breathable waterproof membrane which helps the heat to pass easily through.
The first is our Allergon Waterproof Mattress Protector, the second is our EcoSleep Waterproof Mattress Protector. The only difference between the two is that Allergon uses a cotton terry upper surface which is soft and absorbant. EcoSleep has a flat woven surface, like a normal fitted sheet so can be slept directly on, this surface is made from Tencel a natural fibre that is stronger, softer and much more absorbant than cotton. You can see them both using the links below.