Importance of removing ripples in output of dc-dc converter
It's not possible to make a power supply of any kind with no ripple. The ripple is made smaller until it's acceptable by improving the regulator or the filtering.

All of your questions depend on your load and application. If the load is a resistive heating element, you can ripple all you want: the resistor will still get hot. If your load is an extremely sensitive measurement instrument, an unstable or noisy power supply may couple noise into the measurements, which may be unacceptable. If your load is an ordinary microcontroller, a little ripple or slightly poor regulation may be fine; very bad ripple or very poor regulation may make the processor unstable. The microprocessor's datasheet will specify how good is good enough.