Bad Waveform on Output of Buck Converter
What you're seeing is a normal switch node waveform in discontinuous mode.  

The inductor current increases during the switch ON time, then ramps down during the OFF time.  If the inductor current reaches zero before the next ON time, the inductor will ring with the capacitances on the switch node.  This is the sinusoidal damped ringing you see on the switch node. Image from  here:



If you increase the load on the output you will see the converter transition to continuous mode and the waveform will look like a square wave without the ringing, since the inductor current will never reach zero image: