Why is this photo not super sharp?

[KmH]

(simply a limitation from your aperture, not the lens).

It is indeed a limitation of the lens.

To keep the price in the consumer lens range Nikon had to compromise on the quality of the lens elements used, the precision of the figuring of the lens elements used, and on the design of the lens elemntholding structure.
At NikonUSA.com ypou can see the an illustration of the lens elements inside the 35 mm f/1.8G.
You can see how the front lens element is a single piece of glass that has a rather complex shape.

Another issue not mentioned is the anti-aliasing (AA). or low-pass filter that is in front of the D3100 image sensor.
the AA filter softens focus to some degree. The aggressiveness of an AA filter varies from camera model to camera model.

Nikon's newer cameras, like the D800 and D7100, have a new type of low pass filter that minimizes anti-aliasing, but make moiré more possible.

 

[jake337]

I'm having trouble figuring out how to post a full size version I guess, but as far as original and unedited, this photo hasn't been touched. Just uploaded it from my camera to Flickr.

What do you have your sharpness set to in-camera? If you are not doing your post processing on a PC then you are letting the camera process The jpeg.