You may regard it as tacked on. As a GM, I regard consequences for actions performed - including mocking gods or dead wizards - as one important way in which the fiction matters to the game, and the game therefore plays differently from a boardgame. (Consider it a variant of the "reactive dungeon" approach of using the fiction to supplement the purely mechanical reasons for deciding whether or not to rest.)
At
post 228 I said that if you want to use Vicious Mockery to fight a skeleton or ooze or other emotionless being you don't mock it, you mock its sustaining principle - its creator, the magic or demon prince that underlies it, etc. I am still saying that. But the mockery in question doesn't harm the creator, the magic, the demon prince etc. It harms the ooze or skeleton, perhaps by reducing its sense of self, or its will to act (in the form of psychic damage).
That may be an issue (=problem) for you. It is an opportunity for others, to build their own narration around the oucomes and parameters dictated by the mechanics.
I think it does, though. Look at the art. Look at the example fiction in the *Power books. Read Tolkein. Read Doctor Strange. I think most players of D&D have a pretty good expectation of what will happen in fansastic fictional situations. The game mechanics assume that those expectations will be drawn upon to shape the fiction around the possibilities that the mechanics permit.
Whereas I don't see why it matters that different tables - eg mine compared to Mallus's - might do this differently. At my table, a burst that targets all creatures also hits objects. I'm sure if Mallus were to join my group any initial confusion would be quickly allayed or negotiated away. And likewise if it were vice versa.
I don't have the 2nd ed PHB. I do have Moldvay Basic, which I referred to. (I've reshelved it, but I think the page was B10. I'll also take this opportunity to mention that, according to the Moldvay rules, Fireball targets creatures in the burst, yet in playing Basic D&D over 25 years ago we were able to come up with the idea of using fireball to set fire to wooden structures.)
Again, this is in Moldvay Basic. From what Mallus says, 2nd ed PHB has a longer list that includes at least Hercules.
Which suggests to me that the capabilities of actually strong humans shouldn't be used as the benchmark for "realism"/verisimilitude/genre-aptness.