Actually, all of them do smell due to the combination of chemicals and bacteria in their bodies. It’s just that some grimer and muk’s bouquet of aromas are more subtle than others’, and in any case, the longer you spend with a grimer or muk, the less you notice their stench.
Speaking of which, it’s true that grimer and muk with the Stench ability possess more potent aromas than those without, to the point where they’re able to weaponize it (as opposed to making use of their sludgy, amorphous bodies or their toxic composition, as with Sticky Hold/Gluttony/Power of Alchemy or Poison Touch, respectively). These pokémon are indeed harder to become acclimated to, so whereas the scents of Stenchless grimer and muk are actually rather bearable after a while (perhaps even pleasant to a number of people), the scents of those with Stench may be a different story.
Nonetheless, though, I agree that grimer and muk are a highly misunderstood pokémon, and even if they do possess rather odd aromas, this should be no deterrent to a human trainer.
Just … don’t handle them with bare hands. Trust me on that.