To clarify, responses do accept wikitext, but MoodBar comments do not.

Accepting wikitext in MoodBar comments is relatively low priority because the feature is focused on enabling new editor feedback. Most new editors aren't familiar with wikitext to begin with, so they tend not to use it.

I understand the rationale for saying this isn't a high priority, but I suspect the argument could be made that newbies who have just learned about wikitext would naturally try to use it in MoodBar and be disappointed when they find that it doesn't work.

I have an almost ready patch for this, but lets have some more comments ;)

https://gerrit.wikimedia.org/r/#/c/49474/

Change 49474 had a related patch set uploaded by Qgil:
Parse feedback as wikitext.

https://gerrit.wikimedia.org/r/49474