Reducing Voltage Ripple in a Switching Converter.
Your output ripple will be a triangular voltage waveform at the converter switching frequency; the triangular current waveform given to you by the output inductor superimposed onto the output capacitor ESR. What you have on your scope is huge CM (common-mode) pickup which is swamping the actual ripple.


Turn your scope bandwidth limiter on. Your true ripple will be orders of magnitude lower than 20 MHz.
Measure directly across your output capacitors with a short probe to minimize pickup.
Use your scope cursors to ignore the junk at the switching transitions and just measure the triangular part of the waveform.