It really depends. While doing BJJ, if I was mounted I would sometimes give my back since I felt comfortable I could defend the choke and regain guard. It can be easier at times than trying to escape or sweep from mount. In MMA, giving up your back can be justified because A) You're both sweaty so it'll be harder for your opponent to keep back control. B) With the gloves on it's harder to finish chokes and C) You're tired of getting punched in the face from mount.
Obviously, the downside is if you're fighting high-level MMA, there's a good chance your opponent has world class BJJ, so giving up your back on purpose comes with a huge risk, but I wouldn't call it a straight "I give up" move. Now turtling up on the other hand...