No, it isn’t. Check the tapes yourself:
Spinal’s backdash has a lot of use because it is average. I’m not arguing it’s one of the best in the game (it isn’t), but neither is it bad in any meaningful sense of the word. It recovers in 24 frames and leaves him within range of all his best buttons. If you’re worried that it doesn’t take him far, don’t be - most backdashes in KI don’t take their character beyond the opponent’s effective range either.
There are 10.5 characters with backdashes that recover faster than Spinal (warrior Eyedol is faster, mage Eyedol slower). There are 10 characters with backdashes that recover slower, and 5 characters with equivalent recoveries. The characters with backdashes that are better are better by 6 frames at most, with the vast majority of them (8) only being better on recovery by a frame or two. Among the characters with worse backdashes, 5 of them have backdashes that take upwards of 30 frames. There are obviously quirks in this tabulation (Cinder’s backdash takes 30 frames, but is definitely one of the best in the game, instinct TJ has best in class mobility, etc), but it does a good enough job of making my point. Namely, there is a definite tier of crap backdashes in Killer Instinct; Spinal’s is not one of them.