I think I accept this but had a "wait, what?" moment in one of my games this week.  Party took down a Young Remorhaz using a mix of mounted rider baiting and at range harassment with long bow, toll of the dead, vicious mockery and dissonant whispers (the last one was cast at very close range and was literally the last word of the fight).  When the Bard proposed using vicious mockery to do her part in the fight, I double checked the spell description and it said it affected creatures and there's no barriers I could see for a Remorhaz taking psychic damage.

After confirming the RAW, I wasn't really bothered by it, and an older member of the table made a nod to the line in Aliens when the Marines were told to dump their ammunition "What are we supposed to use, harsh language?" and the whole party dug it.  I guess it surprised me because I always associated (inaccurately) spells like vicious mockery and dissonant whispers with sentience/sapience, I just never thought of it being used on something more beastial on the monster spectrum.  I know the bard in question is going lean more on this area of her spell list, so I just need to be on the look out for creatures that can not hear and or immune or resistant to psychic damage.  Also thinking of creating a magic item, a Mirror of Affirmation, where a attuned user beholds their mirror daily and says "I'm good enough, I'm smart enough, and gosh darn it people like me" and is granted resistance to psychic damage, maybe even immunity to vicious mockery.  Not to "get" this Bard, but I like the idea.  Now I got to figure out what degree of resistance is worth an attunement slot. 

Anyone can insult someone. I feel like a Bard with Viscious Mockery has tuned into a common band of communication shared by all creatures. They don't know what you're saying, but they feel the intent. Getting that intent across is what bard is all about IMO.

Yeah, on further reflection (before I started engineering the mirror) I'm aware that IRL animals can be emotionally abused, etc.  So I'm on board with it.  And yes Vicious Mockery to me is the Bard magically honing their remarks to be particularly, perhaps incisively cutting.  It's still magic, but magically predicated on the Bard's mastery of the power of language/communication.

Spells are supposed to be said in a loud, audible voice, but they do not require the target to hear them, bard spells are based on insults, but have been converted into magic, so the state of the opponents cognition isn't really relevant. They might as well be immune to insult spells because they're asleep, and can't acknowledge them. 

The idea of spells being spoken aloud to harm opponents comes from Cursing. In this fantasy setting, these audible commands of harm become a reality because they bend the weave toward an effect, not because they are received a particular way by the target. If you want to homebrew a different version of fantasy other than the games lore, have at it, but there's a very rational and detailed explanation available in the games existing lore.

Via magic, you can literally talk to 3 int animals and 0 int plants, the magic provides the target with meaningful communication and useful reflection on information it couldn't generate in its own, nor language it could access or understand otherwise.

I mean, Vicious Mockery works on sapient creatures who don't understand the first word of the bard's language as well. Actual, literal, understanding clearly can't be necessary - it's just more fun for those who do understand.