I have read that it is always desirable to create a “child theme” so that any changes I make don’t get washed away if the main theme is updated.
However, one of Jetpack’s features is a CSS editor that lets me override the theme’s default style.css. Therefore the original style.css is never edited.
Does that provide enough protection against the updating problem so that creating a child theme is no longer necessary?
Depends on what changes you’re making. If you want to make functions etc, then no.
For the most part, I find it works great for when I want to tweak a child theme of a framework.
Yes, as the CSS is being handled by a plugin, updating the theme will not override this settings.
Modifying the CSS is not the only reason why a Child Theme is necessary. Although a properly formatted style.css is the only thing you need to create a Child Theme, there are many other possible modifications:
functions.php
template files
Internationalization.