Why can higher voltages lead to less efficient electroplating?
When you apply the required voltage for the desired cell, then your desired product is being produced, but if you provide an overpotential then you forced the other reaction to also occur(the undesired one).

For eg. If in a reaction two species can be oxidized then, of the required potential is applied then you will get the required product but if an over potential is applied then the other species will also get oxidized and you will have two oxidation products.