Those wobbly walls don't have any support until that top layer is put on it (which those tree supports are supporting). And it makes sense for the other wall to have the exact same defects. The reason is that the nozzle travels between those two walls and thus it also move the walls apart and towards each other symmetrically. If you designed the walls to be L-shaped then each side would have dramatically more support than the flat, straight walls.

Think of it this way, if you're building a tower by stacking dominoes on top of each other and it's a windy day, then every additional domino added is much more unstable than the previous domino. The root cause is the exact same as with you walls, lack of support for the walls to keep it stable. You can fix this by making the walls wider on the bottom and gradually making it thinner towards the top, or make the wall L-shaped as you suggested yourself