Both a cooling topper and a cooling mattress pad will help to mitigate issues with sleeping hot overnight. If the heat issue is excessive, consider using both a pad and a topper, potentially swapping out bedding as well.
Along with added cooling, a mattress topper can be used to transform the feel of a mattress to make it softer or firmer, for example. If your mattress is lacking in plushness or support, a topper can be used to add a bit of relief as a shorter-term solution than purchasing a new mattress.
If you do not need to make any changes to the feel of your bed, then a topper may be unnecessary. A mattress pad may suffice to provide a bit of extra cooling capability to the bed.