Hi Km,
I don’t know which mattress you are looking at so you would probably be in a better position to tell than I would by looking at it or lying on it.
Most of these types of questions will have a “range” of answers that aren’t black and white depending on the specifics of the mattress. In very general terms though a loose fitting, thin, or stretchy cover would “allow” the latex layers underneath it to conform to the body shape more closely. The latex layers underneath the cover have much more to do with support and whether you “roll together” in the middle than the cover itself.
Tighter fitting covers that don’t stretch (or mattress protectors or sheets for that matter) that fit too tightly can create a “drum” effect that can also restrict the ability of the latex to compress. On the other hand … a cover that is very loose fitting can stretch and there may be some wrinkles on the sleeping surface which is more of a cosmetic issue than a performance issue.
If you have split layers (a separate layer on each side of the mattress) then it’s usually a good idea to have a quilted cover or a thin layer of material over the “split” top layer to even out the different firmness levels on each side and prevent you from feeling the split itself (see post #2 here).
While a very loose cover can increase the chance of any shifting … latex is a very “sticky” material so it’s not likely to be an issue even with a loose cover and if they do shift then it’s easy to unzip the cover and “wave” them back into position (see post #2 here).
Phoenix