Hi Alamib,
The thickness of a mattress is really just a side effect of the design and is only one of many variables that can affect the feel and performance of a mattress and by itself isn’t particularly meaningful (see post #2 here). In some cases higher weight ranges will sometimes do better with a mattress that is a little thicker than lower weight ranges but even this depends more on the specific design and combination of materials in the mattress and on how well the mattress “as a whole” matches your specific needs and preferences than anything else.
In other words … there isn’t a specific thickness that is inherently better or worse because it always depends on what is inside a mattress rather than on the thickness that all the individual materials and components in the mattress add up to.
Phoenix