Allowable ripple voltage
There is no one answer about how much ripple is too much, because that depends on the devices being powered.

Yes, LDOs don't attenuate high frequency ripple much, but much of it can be filtered out before the LDO with a chip inductor in series followed by the LDO input cap to ground.

All that said, 20 mV ripple on a 3.3 V supply should not cause trouble for an ordinary microcontroller or digital logic.