Hi Benemme,
Post #89 here should help with choosing a mattress protector that has the combination of qualities and tradeoffs that are best for you (I’m assuming a protector is what you mean because a mattress pad is thicker and different from a protector and will have a bigger effect on the mattress)
The most popular protectors are the thinner versions with a semi-breathable waterproof membrane that have little effect on the mattress but some of these can create a “drum” effect which can make the mattress feel firmer. a thicker mattress pad will also affect the mattress more than a protector.
Phoenix