It depends. Tumbling prior to impact is generally not good because it will seriously degrade accuracy. But, if it doesn't tumble until after impact, it might be OK -- depending on whether you wanted it to do that.
The problem is that designing a firearm to induce tumble upon impact at a certain range means that the bullet will be unstable at ranges longer than that.
So, there's no single answer, unless you specify the constraints of the question.
Only if the projectile tumbles before it hits the target.
That is why the mountain rifles in the revoulutionary war gave out boys an advantage over the british with their (unrifled) muskets
Rifling, and centrifugal spin of the projectile, forces the projectile to travel straighter, whereas if it tumbles it can go anywhere depending on what axis it decides to tumble on
I tumbling round does not indicate a malfunctioning firearm, per se. For instance, the shooter might just be using the wrong bullet weight for the amount of barrel twist. Like I mentioned above, the military originally used a 62 grain bullet with a barrel that had a 1:9 twist in their M-16 and derivative rifles. That was changed a few years back to a 55gr bullet with a 1:12 twist. If you mix and match the barrels with ammo they weren't designed for, you'll probably get tumbling, but they work fine with what they're designed for.
It could also just be a poorly design firearm. Since one of the primary rules of judicious firearm use is to know your target and what lies beyond it, if the weapon does not allow the shooter to know the bullet path, I would say that's a malfunctioning arm.