Tremor isn't an anti-air move. For jump in's you are going to want to use vortex punch (preferably heavy because it has a better hitbox, but medium if you don't have time for all the startup frames.) You can still recapture off a vortex punch, so it has the same effect of getting a combo like Tremor, but actually works as a functional anti-jump-in.
Think of Tremor as a spike/meteor smash from super smash bros. It isn't a anti-air move, it's a way to punish a predicted jump (usually because they are trying to get back on the stage, making their jump predictable) and requires you to therefore be over the place you predict they will jump to.
Don't worry too much about catching them jumping backwards- let them hop all the way to the corner. Tremor should be catching neutral jumps intended to punish you when you try to powerline or grab them with chute toss.
If you do catch a jump back with a Tremor, your timing should be close to getting them on tripguard as they land. This timing is actually similar for catching them if they neutral jump as soon as you start to do a powerline. Catching them near the time they are landing means you are over them rather than at equal height and you won't be hit out by light air attacks.
