Hi bedtoosoft,
The deeper layers of a mattress are the layers that provide “primary” support (which is what most people mean when they talk about the “support” of a mattress) and the upper layers are the layers that provide pressure relief and “secondary” support and much of the “feel” of a mattress. There is more about the differences between primary support, secondary support, and pressure relief and “feel” in post #4 here and the posts it links to.
In most cases the reason for lower back pain (which is the most common symptom of a mattress that is too soft) is because the comfort layers are already too thick and/or soft and this can “allow” the pelvis to sink down too far before it is “stopped” by the firmer support layers in the mattress and this puts the spine out of alignment. If you add additional thickness on top of the mattress then even if it’s firmer than the layers below it it still wouldn’t correct the fundamental issue of the comfort layers being too thick because you would be adding even more thickness and you would be even farther away from the firmer support layers of the mattress. Adding a topper to “fix” the support of a mattress is always risky because you will end up with the softer layers that were on top of your mattress being even deeper inside the sleeping system and you could make the problem worse.
In some cases though it can be worthwhile trying a latex topper (or other materials as well) on a memory foam mattress that is too soft because a topper can reduce the amount of heat that reaches the memory foam which can reduce the amount it will soften or the time it will take to soften with body heat and this may be “just enough” to make a difference with the support but again these types of changes are always risky and what can work for some people may make things worse for others depending on the specifics of the materials and the design of your mattress.
A latex topper would change the “feel” and the “mushy feeling” of memory foam layers that are too thick and soft and will give you a more “lively” sleeping surface but it may not have “enough” effect on the support of the mattress because the softer layers underneath it will still be compressing to some degree and the latex will still bend into the softer layers underneath them.
The only way to know for certain whether it would make “enough” difference for you would be based on your own sleeping experience and in cases like this where the results are very uncertain then the return policy of any topper you purchase would probably become a more important part of the “value” of your purchase just in case your “real life” results aren’t what you hoped for.
Phoenix