Mood is expressed in multiple feeds currently published, though not always in consistent form.

MySpace exposes mood with both a word and an image

LiveJournal allows users to add moods to their posts, but does not include them in the feed. They have many word-based choices and flexibel icons to represent them.

GetSatisfaction allows users to add expressions of mood to their comments: see example

In addition, there is the long-standing convention of using textual smileys to a comment to express mood :) 

A possible way to represent mood in Atom XML or JSON could be to use emoticons.