Is it OK to put a sleeping MacBook pro (Mid 2015) in your backpack and carry around?
The mid-2015 model is only available with SSD. I.e. you do do not have a hard drive with moving parts to worry about.

It is therefore safe to carry the MacBook Pro in your backpack and carry it around.

Just be sensible: don't drop the backpack, don't sit on it, etc.