Hi kiquali,
The Genius is rated as the firmest of the iComfort mattresses and would typically be too firm for most side sleepers who need more cushioning under their pressure points. I think that you may have chosen a mattress that isn’t suitable for your body type and sleeping position so the first suggestion I would make if it’s possible would be to choose a mattress that is more suitable for your body type and sleeping position.
All foam materials will break in and soften to different degrees in the initial weeks or months of use and this will be followed by a more gradual softening over longer periods of time and then by the breakdown of the material itself (see post #4 here).
A bigger issue though with the iComfort Genius though is its construction. The layers include …
2.75" of 4 lb gel memory foam
2" of 1.3 lb polyfoam (this is a very firm foam that is part of why the mattress is so firm)
6" of 1.5 lb polyfoam support layer.
Serta rates this as a 3 which is the firmest mattress in the iComfort line (although some people “feel” that the insight is firmer) but a big part of the firmness comes from the very firm but low quality 2" layer of polyfoam under the memory foam. Because this is very low quality/density material it will soften and break down relatively quickly (compared to higher density/quality polyfoam) so if a mattress has been in a store for a long time it will feel softer than a new mattress and a consumer will receive a new mattress that feels much firmer.
On the other hand if the floor model is fairly new then the new mattress that a consumer receives will feel close to the same but over time it will soften and break down relatively quickly and can develop “virtual impressions” or soft spots that aren’t visible and aren’t covered by warranty.
The problem with foam softening of a deeper layer like this is that the softening will happen unevenly and will be more under the heavier areas of the body so it can lead to the loss of support under the pelvis and alignment issues (such as lower back discomfort and pain) and it may not soften enough under the lighter parts of the body that need more pressure relief (such as the shoulders for side sleepers).
The use of lower quality materials that will soften or break down more rapidly is one of the biggest issues with the major brands and the reason I suggest that they all be avoided (along with any mattress where you aren’t able to find out the quality of the materials inside it).
Mainly because “marketing stories” are what sells mattresses and the larger manufacturers don’t have any real interest in providing more meaningful information about the quality of their materials to consumers. In most cases the smaller manufacturers who are more directly accountable to their customers and who “want” their customers to be able to make meaningful comparisons with other mattresses are the ones that will “educate” their customers and provide meaningful information about their mattresses that allow their customers to make more informed choices. You can read more about this in this article and in post #12 here.
To get to your main question though … if you can’t exchange the mattress for another one that has softer and more pressure relieving comfort layers and hopefully better quality materials (or get a refund and start all over again looking for a more suitable and better quality/value mattress) … then a topper can be a good choice for a mattress that needs thicker or softer comfort layer and additional pressure relief as long as the mattress is still in good condition and doesn’t already have any soft spots or impressions in the sleeping surface.
If you do decide to go in this direction then the topper guidelines in post #3 here and the posts it links to can help you decide which thickness and type of topper may be most suitable for you.
Phoenix