Jax is really, really weak right now, and he has been for a LONG time.
Even when he was good he was weak, because his design is leftover from a different time, when dodge was still a thing, and thus he could be MUCH tankier than he can be now, in his own unique and interesting way.
He needs a kit rework probably, or at least an update. His current design basically forces him into the splitpush-only role unless he's super fed or the enemy team doesn't know how to deal with him(the same problem Yi has).
That's not to say he can't be OP if overbuffed, but that amount of buffing is unlikely to happen before he gets a rework//update imo.