What is the source of distortion in this simple common-em. Colpitts oscillator waveform?
This is a poor way to make a Colpitt’s.  The Q is too low on the output as the current source collector turns into a switch from high to low R.
The better configuration re-arranges the parts so that the inductor is on the collector to Vcc and there are 3 caps from supply to collector to emitter to ground that  ought to be around 3C,C,3C respectively for gain.
Emitter voltage determines the quiescent current from the base to prevent saturating the collector with the appropriate &lt;50 mV AC across Vbe to pump the current. This gives almost 2x Vcc output swing and approaching saturation dampens the Q but is much easier to avoid distortion than your circuit.  I have a simulation in other answers if you wish to search me.
Your way (more power hungry to get higher Q)
My way. More signal, more efficient, higher Q across collector.