Jax isn't a bad character, by any means, I'd say he's upper mid tier. However, he's extremely strong in the corner, Kabal can only run away for so long - Jax gets to capitalize on this big time. I do think his unblockable ground slam is a little broken, but so is Kabal, so it's whatever in that department. The corner is just death with Jax.
It doesn't make Jax the best character in the game, but he's a strong anti-Kabal character. Remember, this game is all about tools. CD Jr is an extremely strong player as well.