A commoner is not a wizard. A commoner will not wear plate mail or anything beyond studded leather or maybe hide. In fact, a commoner might not even see a gold piece in his entire lives. The hammer your commoner might wield will not be a warhammer but a carpenter's one. At best a spear or a bow. Monsters will judge by the standards they meet, the adventurers. And since adventurers tend to emulate their heroes my point stands.
Maybe they won't, but a guard or a soldier isn't much better in terms of a challenge, and they certainly won't cause a thunderclap explosion.
So, now we already have a divide to take into account, there is the Dwarven non-adventurer and the Dwarven Adventurer. So, now tell me, once they have determined that these are adventurers how can they tell which people calling down death and explosions of raw power are to be expected and which ones are unusual and frightening? Is the Goliath charging them with a greatsword while the ground catches fire beneath them just ho-hum, or something to be feared because it is unexpected? What about that elf over there surrounded by the spirits of the dead who just killed two of your buddies with a single strike?
And what point is it "business as usual" compared to "strange and terriyfing"?
And what if, in your example, the armored dwarf meets the melee monsters with a wall of fire?
Then that is perfectly understandable and expected, doesn't even require the dwarf playing against type. In fact, dwarven heroes would likely often have used wall of fire.
Or your little halfling lifts the heavy orc and throws him into the the fire pit?
Is that really so unexpected? Doesn't take much to do that. Very doable by a halfling with 12 strength, which isn't even that strong for a halfling. Besides, is it any less terrifying than a dwarf throwing your friends into a fire pit?
Ho the dwarf is strong and meets the foe with a big bong of a big whammy hammer... nothing new. The strength and power of the blow can be surprising, but not unexpected. That same dwarf with a fire wall/ball has both and would force monsters to reconsider what they took for the absolute truth and could potentially make them make mistakes. Or seeing the strength base halfling grabbing the orc to throw it into the fire pit or clinging with one hand on the dragon's back and proceed to hack it with unsettling rage with his big... short sword.
I see, so a Dwarven artificer Battle Smith with a 16 Intelligence using Booming Blade is completely expected, of course they can do that. But a Dwarven Cleric is strange and confusing and will cause them to question their choices.
Aren't... clerics very archetypical choices for Dwarves? I swear I've heard that somewhere.
And like I said, that halfling wasn't even that strong. And clinging to a dragon's back could just as easily be a dex based halfling.
These are the images that playing against type can bring. Do we see those very often? Nope. But when we have one, we are happy because it is both unexpected and refreshing. But if every members of every races can do anything, it quickly fades into the usual. Other systems have gone this way and they are not even near the heel of D&D in popularity.
Or playing with type it seems. Since there are multiple routes to get to the same place.
Again, if you decide to have your monsters meta-game, that's on you, but it seems like you are just arbitrarily giving a boost to people who are unusual for no reason other than you didn't expect them to pick that combo.