Gap between top mount sink and new counter
If it is warpage (that should be a word)... you can sand out that area given this isn't stone.   You could sand a groove for that and set the back into it.  I am just look at a picture but it looks like the back of the sink has a little bend on it (is the other side look exactly the same?).   The first 80% seems straight though.
So I am not sure if that bend is causing it not to sit flat or if there is something next to the sink that is protruding where it shouldn't be... or both.   Can you wiggle the sink around?   If it is ultra tight your cutout may need to be a little bigger.
Just FYI - fire clay sinks always have small deformities.   Most too tiny to notice.  I did get one where the bottom was jacked and basically at a 15-20% angle - you know where you connect your plumbing.   I had already secured it to the top weeks before and had to hit it with belt sander for about 20 mins before it was flat enough to not leak.   You can sand the sink but I personally wouldn't given the picture here.
In the end this isn't the end of the world.   The gap isn't humongous - yes bigger than normal or bigger than you want.    When caulking this (clear silicone) fill in the voids on the first pass.   Let it dry a couple hours.  Then go through with finishing layer.