Kit lens is (often) worse at the zoom end, and is (often) worse wide open. So both are at play here :).
As someone else said, his focus was pretty close (if the recorded focus distance is correct, I can’t find it?), so only things closer to the camera are properly in focus.
I’ve opened the picture now and I’m clicking around in it.
- The green trees at the very bottom, and at like 1/3rd on the left, are pretty sharp. If you then go up, the leaves are pretty sharp and the darker tree in the middle is pretty clear.
- Pretty much everything on the right side is not sharp and pretty blurry. But it’s also closer to the camera! The trees on the bottom left are still pretty good. They are in the distance.
- Now, I expect the branch that is ‘in front’ on the far left, about top 1/3rd in height to be also blurry. Because it’s pretty close. And it seems less blurry than the top right, for example. But the top right seems closer to the focusing distance, so I expected it to be sharper, not blurrier.
So, I come to another conclusion to be added to all the tips and things mentioned here (which all seem very correct and true!)
- Your lens seems sharper on the left half compared to the right half. I think this is what they call ‘decentering’? (Correct me if this is not correct, I never understood the term).
- Basically, it’s just a defect of your lens. Happens in older lenses (older designs) and cheaper lenses. So the kit lens from on old D3100 can have this, for sure. The kit lens on my very first Sony A100 was also notoriously bad for this. And I have a 80s Minolta AF 16-35 2.8 A-mount lens that is just hilariously bad in this example (but that lens is now 40+ years old and cost me less than 100,- EUR on eBay at some point :P).
Stopping down can (and will) help with this, so that means be more at >= f8 and <= f16 with your lens when using the end of your zoom range.
I notice that your shot is pretty much at the limit of clipping a bit. Nicely shot! (Or does this Nikon have a fancy working highlight-priority mode? 
). What I mean is that you shouldn’t have taken the shot by exposing more (not shooting it brighter). If you want it brighter, do it in software later, but you shot it so bright, that it’s pretty much right against the limits of your sensor. That’s a very good thing! Overexposing makes the sky a hassle, while AFAIK any Nikon old or new can handle a bit of pushing later in software without issues.