You have the antenna itself tuned properly (same SWR on 1 and 40, lower in the middle). Unless something is put together improperly with the mount, and I'm not familiar with CB mounts, I bet a little more grounding will get you into better territory.
I always use serrated lock washers on every bolt I want to be grounded.
View attachment 300279
View attachment 300277
I would put one here on the bottom side of the bracket if you can.
View attachment 300278
I would put one on this bolt. I would also put one on this bolt between the bracket and tub. If there's a nut on the back side of the bolt, I would also put one there. I assume there's 1 or 2 more mounting bolts like this, I would put 2-3 on each of those as well.