Can those on the enemy team get punished for encouraging allies to feed/afk?

Unsportsmanlike Conduct, definitely reportable.

I would say that reporting them for negative attitude would be the best option here. Not sure how the system would react to this kind of language in particular, but at worst it would flag them to look for other punishable behavior.

I would say if they typed it in one or two lines and weren't rude the Tribunal AI won't pick up on it and you're better off not wasting your time and potentially decreasing the effectiveness of one of your reports by false reporting.