I can't say what's wrong with your D5100 pics, but I own both a D5100 and a D90 and I can't tell the difference between images from each camera--both are sharp. I think the D5100 is a great camera.
In fact, the only real difference between the D5100 and the D90 is the convenience of the D90's controls and the clearer view screen. But frankly, given the D5100's larger sensor and its capability of iso 100, i'm not sure why anyone would pay more for a D90 (I bought mine used and paid about the same price I paid for the D5100). I bought the D90 for the flash commander mode but found a radio control trigger works far better, and works just fine on the D5100.