Clipped input sine with 1N4148 signal diode
If I measure directly the input signal shouldn't I see a 50Hz sine
  wave?


If you were measuring the input signal at source then it would be a pure sinewave but you have output resistance set for your generator of a value 50 ohms and this will allow the output voltage of the source to become distorted.

You can prove this by setting Rser to 0 and using an external 50 ohm resistor in series with V1 - now V1 will be pure and undistorted.