softness: me, glass, or camera?
Appears to be mostly you. You haven't told us what AF-area mode you were using and I can't read your EXIF data to find out. But:
#1: Assuming that you're using the center point to focus, it's focusing at infinity and you're getting normal atmospheric distortion and softening of distant objects.
#2: I personally don't like to shoot any slower than 1/125, even with a wide angle lens, but that's to account for my shaky hands. You're shooting into the sun here, and there's a lot of contrast reduction from the backlight. The dead center of the frame appears to be the best focused, but the overall image is rather soft. Was there a lot of mist in the air in this shot?
#3: The camera focused fine, on the tree leaves at the dead center of the frame. The monkey was a bit to the left, out of range of the center focus zone. I'd also advise that you're perilously close to too slow of a shutter speed for the chosen FL. This shot should be taken at around 1/200-1/250.
Here's a shot I took in India a few months ago. It was essentially ruined by atmospheric pollution and heat:
Looks a lot like your #1 shot, doesn't it?
I'm not particularly impressed by the image quality of your Tamron - lots of CA, soft at the extreme FLs...but it's a superzoom and that's what you'll get. However, it is capable of excellent performance away from its FL extremes as evident by #3. And, as I and others have pointed out, Nikon wants you to move the focus point around the frame to lie on your subject, not focus and recompose (it screws up exposure sometimes if you do). If you do choose to focus and recompose, make sure the camera is set to lock focus on the half press. Hopefully you're not using the dynamic area modes, which often will not focus on what you want but rather what they determine is the most contrasty portion of the frame that meets the camera's definition of "subject".