A couple of things -
1. LCD screen on camera is small small size, what you see without zooming always looks better especially if it is a little of focus. On the much much large LCD screen of a computer (which should be calibrated) any defect will be clear seen. Defect meaning on exposure - over or under, white balance setting - colors look weird if this is out, focus - if is not focused it will look blurry, bad depth of field - if use too large an aperture i.e. 2.8, 1.8 for a group shot of many layers of people the picture will look bad cos cannot see clear many of the faces.
2. Most camera LCD screens tend to be too bright vs a calibrate LCD monitor - which view on monitor would look too dark.
3. you have not learnt how to approximate from what you see in the camera LCD to what you should expect in the computer monitor. Try using a cannon 1Ds mark2 's LCD screen, then you will puke blood as it so unsharp that you need to learn how to guess what will be blur and what will be sharp; colors look a little better on camera then in monitor - and the 1Ds mark2 was once upon a time Cannon flagship camera.
4. maybe but very unlikely there is a fault in your camera LCD - try seeing same file in a friend's 600d body if same same then camera no problem. If not same same then problem solved only need to go bang table at either shop you buy at or Cannon repair center.