Hi Kyle_1988,
There is more about the different types and categories of mattresses in this article.
These types of generalizations are rarely if ever true. As sad as it is … most of the members here that have spent more than a couple of hours on the site will know more meaningful information about mattresses and mattress materials than most of the mainstream salespeople that sell them. This is one of the reasons that it’s important to make sure that any retailer or manufacturer is able and willing to provide you with the information listed here before you go and visit them to test mattresses.
There may be hundreds or even thousands of different mattresses that are all in the same category that can have very different designs in a wide range of firmness levels.
If the only issue with a mattress is that it is too firm and there are no soft spots or sagging in the mattress then a good quality topper can certainly be an effective way to add some additional softness, “comfort” and pressure relief to your sleeping system but the only way to know whether a specific mattress/topper combination is a good “match” for you in terms of PPP (Posture and alignment, Pressure relief, and your own Personal preferences) is based on your own careful testing or personal experience on the combination. If you can’t test the combination in person then there is always some risk and uncertainty involved in adding a topper because the specifics of the mattress itself along with your own body type, sleeping position, and preferences can affect which specific topper would be a suitable choice on any specific mattress.
All the layers and components in a mattress will have some effect on all the other layers and components above and below it but the only way to know whether any differences between two mattresses or a mattress and a mattress/topper combination would be enough for you to notice a difference would be based on your own careful testing or personal experience.
Even if a mattress/topper combination has exactly the same materials and components as a mattress and the only difference between them is that the top layer in one is a topper and the same layer as the topper is inside the mattress cover in the other … this alone may be enough to make a difference in how the mattress and the mattress/topper combination feels because a separate topper can “act” a little more independently and “feel” a little softer compared to having the same layer inside the cover (see posts #3 and #4 here and the first page of posts in this topic).
I would also keep in mind that there are also no “standard” definitions or consensus of opinions for firmness ratings and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness could be rated very differently by another manufacturer. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here) so different people can also have very different opinions on how two mattresses compare in terms of firmness and and some people may rate one mattress as being firmer than another and someone else may rate them the other way around. This is all relative and very subjective and is as much an art as a science.
Hopefully you’ve already read the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps most importantly know how and why to avoid the worst ones.
Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort” and PPP that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists (based on all the parts of your personal value equation that are most important to you).
I’m not sure which mattress you purchased but I would also keep in mind that most of the major brands (such as Sealy/Stearns & Foster, Simmons, and Serta) tend to use lower quality materials in their mattresses than most of their smaller competitors that will tend to soften or break down prematurely relative to the price you pay and I would generally suggest avoiding all of them completely (and the major retailers that focus on them) along with any mattress where you aren’t able to find out the type and quality/durability of the materials inside it (see the guidelines here along with post #3 here and post #12 here and post #404 here).
Phoenix