You need to consider several things:
1. Only a small proportion of the package will be in contact with the skin, the remainder will act as an absorber or radiator to the surroundings. You probably need to insulate the sensor so it's only source of heat is the skin and as little as possible is gained or lost from the surroundings.
2. The LM335 is only accurate to 1C which in medical terms is huge. You will need to calibrate the reading for best accuracy.
3. Assuming you are measuring human skin temperature and the person is alive, (not for forensic or mortuary tests) you are not interested in temperatures below say 10C or above 40C. This only equates to 0.3V difference in voltage across the LM335. The 16F877 ADC is 10-bit so each step in measurement is 4.8mV or nearly 0.5C. You would do well to amplify the LM335 output so the 0.3V range was spread to nearly 5V and the lowest temperature voltage offset to zero. This will greatly increase the accuracy for very little extra cost.
Brian.