Hi Sleepytime,
A topper usually works best to add some extra softness or pressure relief when a mattress is too firm and has no soft spots because a topper can just “follow” the dips or soft spots in the mattress underneath it. It may have a partial or temporary effect but in some cases adding more soft foam on top of a mattress where the comfort layers have softened can make an alignment issue worse.
The only real “fix” for a mattress where the comfort layers have softened is to remove the offending foam and replace it with better quality materials.
There are a few other options in post #4 here that may be helpful with a mattress that has developed soft spots or is sagging.
Phoenix