She's a dirty fighter too. "Go for the eyes! break his jaw! hit him low!" That proves that she is a good military leader, but a horrible person... lion.
"A film should engage, not distract. expand, not contract. Inform, not retract. Redefine, not reenact."
I feel Simba66 makes a good point there, Zira seems to be a highly capable and smart leader...but her cruely and fanatical obsession with Scar are core faults that bring her down.
All-in-all, she can't compete with Scar in terms of whether she as good as him as a villain, but she's single-minded, scary, and fanatical enough to be a solid villain in her own right.