Thickness mainly tells you how much material you are adding. It does not prove how cool the bed will feel through the night. First establish whether the mattress needs a comfort change.
When a thin layer fits
If the mattress feel already suits you, a thin protector or textile pad may perform the needed protection or fabric-change job without adding much height. Check the actual padding and barrier rather than assuming every thin layer behaves the same way.
When more loft is wanted
A thicker topper can change cushioning, but it cannot restore a sagging mattress. Measure the resulting height and ensure your sheet, protector and bed access still work. Consider the extra weight and handling effort at larger sizes.
Keep care and cooling separate
Foam cores often require spot cleaning even when a cover can be laundered. Read both labels. Gel, loft and plushness are not evidence of sustained heat removal. If adjustable temperature is the aim, compare a complete active system rather than buying the thickest passive pad.
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 Travelling With a Humidifier: Need, Power and Cleaning Checks, Gel vs Phase-Change Bedding: What the Materials Establish, and Portable Dehumidifier vs Whole-Room Dehumidifier.
To widen the decision beyond this head-to-head, Queen Cooling Mattress Pads: Passive Pad or Active System? and Best Mattresses for Every Sleep Style provide the broader context.