What does the ripple frequency of a buck converter indicate and how does it affect the DC device connected to its output?
What is the desired ripple frequency for buck converters connected to computers, networking device as power supplies?


Ideally you don't want and don't get any ripple so the desired ripple frequency would be zero. In practice you minimise the ripple so that it does not affect the circuits being powered. Frequency is unlikely to matter in your use cases. In very noise-sensitive analog applications; it might.


  Can we judge the quality of power supply from ripple frequency? (buck converter or SMPS adapters) What does the ripple frequency number mean?




Figure 1. Buck converter circuit diagram. Source: Wikipedia's buck converter.

Ripple frequency just tells you the frequency the converter is switching at. In general, the higher the switching frequency the smaller the inductor can be because it has to store less energy per cycle as the cycles are shorter. (This is why switched mode power supplies are so much smaller and lighter than mains frequency transformers.) At some point high frequency losses will become apparent so, as with all designs, the sweet spot is the best compromise between performance, cost, size, efficiency, weight, etc.


  Also can we measure ripple using multimeter?


Generally not. If you have a multimeter with a sensitive frequency meter you might but I suspect that if you can then you have a high ripple and the PSU might be a poor one.