Well, I mean, learning how to talk and do everything backwards is pretty comedic. As for 'generic' its more generic fantasy at this point rather than generic D&D. Not entirely, but becoming such.
I mean heck, the idea of a 'mirror world' is at the very least earliest from
Nine Princes of Amber, which Gygax blatantly stole a lot of stuff from. There is a reason that the SRD very explicitly states what is, or is not, considered WOTC copyright...case in point, even the names like Limbo aren't simply Limbo (because Limbo existed before them), but instead
quote:
The following items are designated Product Identity, as defined in Section 1(e) of the Open Game License Version 1.0a, and are subject to the conditions set forth in Section 7 of the OGL, and are not Open Content: Dungeons & Dragons, D&D, Player’s Handbook, Dungeon Master, Monster Manual, d20 System, Wizards of the Coast, d20 (when used as a trademark), Forgotten Realms, Faerûn, proper names (including those used in the names of spells or items), places, Red Wizard of Thay, the City of Union, Heroic Domains of Ysgard, Ever-Changing Chaos of Limbo, Windswept Depths of Pandemonium, Infinite Layers of the Abyss, Tarterian Depths of Carceri, Gray Waste of Hades, Bleak Eternity of Gehenna, Nine Hells of Baator, Infernal Battlefield of Acheron, Clockwork Nirvana of Mechanus, Peaceable Kingdoms of Arcadia, Seven Mounting Heavens of Celestia, Twin Paradises of Bytopia, Blessed Fields of Elysium, Wilderness of the Beastlands, Olympian Glades of Arborea, Concordant Domain of the Outlands, Sigil, Lady of Pain, Book of Exalted Deeds, Book of Vile Darkness, beholder, gauth, carrion crawler, tanar’ri, baatezu, displacer beast, githyanki, githzerai, mind flayer, illithid, umber hulk, yuan-ti.
Note that 'Mirror of Opposition' is not on there. Why do you think they changed all the spells that had things like "Bigby's Clenched Fist" to "Clenched Fist"?
As for a more defined fantasy setting-Sure.
The Mirror of All
In the beginning there was creation, and creation was divided when the Master Make created a mirror. At first all of the shards seemed alike, but when the Master's light shone down upon them, one was proven to be the inverse image of the other. One by one the shards fell away until there were soon to be none left. Desperate to save something of creation, the Master bound the one to the other, through which special mirrors were able to bridge the gap between the worlds.
All magic in this world is based on mirrors, to create new objects from thin air, to scry, to travel or to reflect an enchantment back on one's enemies. For a short time, the creations of these weaker mirrors can draw items, creatures or beasts from obscure places that barely Are, but the most powerful mirrors are those that can summon forth the Other. But like matter and anti matter, when a being from the Mirror world enters the True world, he has little choice but to kill his other.
The mirrors do not work both ways.