If I'm carrying a pack, yeah, it's the the middle or lower part of my back that gets and stays the sweatiest.
Without a pack, I believe the region is now called the "taint" =the perineum, the region of the human anatomy between the genitals and the anus as in "taint an asshole and taint a pussy." (from Urban Dictionary).
Wicking fabrics help you stay dry in both regions. Polypro, nylon, etc. On my back, that works really well. But on my taint, cotton consistently is less funky than synthetics. YMMV.
Above about 25 pounds, this is a big reason why I use a framed pack. I hike so much cooler, that it is worth the 2-pound extra weight compared to an internal-frame pack.