A body pillow cover changes the fabric around the pillow you hold. A mattress layer changes the surface underneath you. Neither passive textile should be assumed to cool the entire bedroom.
Locate the uncomfortable contact
If the body pillow alone feels clammy or rough, changing its cover is the narrower intervention. Check the pillow’s dimensions and the cover’s care label, not just a generic cooling claim. If warmth is beneath your torso or legs, a pillow cover will not reach that area.
Avoid changing firmness unnecessarily
A thick topper can change the mattress feel as well as adding another layer. If firmness already suits you, first compare lighter covers and sheets. Powered bed cooling is another category with equipment and maintenance, not an automatic upgrade from a pillowcase.
Test one change at a time with the rest of the bedding kept similar. A cool first touch can be pleasant but does not establish an overnight temperature reduction. Buy for the specific contact problem and check return terms before opening.
See the cooling mattress pad buying guide for manufacturer-linked examples and the differences between fabrics, toppers and powered systems.
See Also
If you are still weighing both sides of this matchup, keep going with Pillow-Top Cooling Pads: Extra Cushioning or Less Heat?, How to Choose a Pillow: Size, Support, and Fill Materials, and HEPA Air Purifier Filters vs Washable Pre-Filters.
To widen the decision beyond this head-to-head, Cooling Layers for Stomach Sleepers: Check Softness First and Best Mattresses for Every Sleep Style provide the broader context.