According to 2011 Moderator Election - Town Hall Chat Digest, the answer is yes. And here I thought my memory was going! But still, making gratuitous changes is rather childish, and forces me to delete the comment entirely (if I notice it).
Yes it is possible for moderators to edit comments. In practice this is normally done only if the commenter requests it (as they've spotted an error but have missed the editing window) or where there is a typo that is clear and correctable. (Both cases are rare so I can't readily point to an example, though I know I've corrected things for people on occasion; I normally let them know this has been done.) Far more common is simply deleting comments, for example ones that are no longer relevant (e.g. 'Please add a MWE' after this has happened).
Yes, moderators can edit comments. The other answer lists a few cases in which this is done.
As a moderator on another site, I've also edited comments to remove rude/offensive content if the same comment also contains useful information (e.g. in a debate in the comments). Similarly, comments can be edited to remove personal information (such as an e-mail address) while keeping useful information.