Should a mattress pad be placed over a mattress topper?

If you are referring to a mattress protector or a cover, there should be no problem as this will help prolong the life of your mattress and the mattress topper. A mattress "pad" and a "topper" are basically the same. They are thin foams placed on top of the mattress to improve sleep comfort or hide the indentations in the mattress. If are adding another thin layer of mattress pad, just make sure it suits your comfort preferences well.