How come my pictures are not well-focused?

Lately, when I go to take pictures with my Nikon d3100, they appear crisp through the viewfinder, but when I view them on my computer, they appear out of focus. It's not awful, but it just makes the pictures look bad. I know that the d3100 isn't the highest of quality, but it should produce pretty good pictures. What do you think?
Thanks
-Sarah

Added (1). Just wondering, is it me, or the camera?

Several things come to mind which you might check.
1. Your viewfinder is not adjusted to your right eye.
2. You may have changed resolution- ALWAYS use highest resolution.
3. You may be shooting at too high an ISO for the subject matter. Example: I use ISO 200 for most of my hand held shots (I'm age 70 and not as steady as I once was). I feel an ISO of 125 is sufficient for younger people.
4. And last but not least, you may have gotten out of the habit of remembering you fire a camera like a gun, that is you hold your breathe momentarily, and squeeze the trigger/shutter.

Have the computer images changed over time? In other words, we're they crip before and are less so now? This would suggest that you'ved, somehow, damaged your lens. It could have been dropped, bumped, left out in the heat or incurred some other kind misuse.

However, if your issue is that your images have ALWAYS been that way, then it MAY be possible that the camera is defective. But be aware that the image in the viewfinder is very small, compared to that on a full size monitor. The flaws you see on your computer may be just too small to see in the much lower resolution screen. Are you certain you have set the camera to take the highest resolution pics possible? If you take extremely low res pics and display them on a large screen, they will look bad. And you won't notice that in the viewfinder. It won't chage, whatever the resolution settings.

Do a few tests. Take several pics of an object, with the sharpest focus you can achieve in the view finder. Then a few shots with the focus a little bit off in either direction of the same object. Mak notes of which image numbers were taken. Now compare them on your monitor. If any of those shots you took at slightly off focus in the viewfinder are now sharp and clear, then you might have a problem.

At best, this test is crude and may be inconclusive. But it might lead you to a solution. In any case, don't attempt a repair on your own. Take it to an authorized camera repair shop. At some point, you may have to decide if it's worth the cost of repair or should just get a new camera.

Actually the D3100 can produce exceptionally high quality images

Here is an image shot with a 10 year old Nikon D100 which has a 6 mp sensor. Your images should be this good or better.

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Check the viewfinder diopter and make sure it is set properly

Next when you focus, make sure that the electronic rangefinder dot on the bottom left side of the viewfinder is a steady green. If it is blinking, that means you are not quite in focus. If it is off, it means that you are certainly NOT in focus

The simple and most common Answer- You need to watch your shutter or aperture settings. You're more then likely just shooting to slow which in return is making your photos seem blurred.