I aggree with OP. At the very least it's de-humanizing and elitist. Furthermore, it's counterproductive. If you've got a villain pegged as a "fish", you may not see it when he manages to pull a move on you and rakes a pot. "He just got lucky" you'll smugly tell yourself. And who's really the tool?
I think this attitude is more of an online thing. Online, they're just pictures on a screen, little avatars, so it's easy to think of them in such condescending terms.
Live, you wouldn't do that, because when you think in those terms, your demeanor shines through. People generally know when you've got a low opinion of them and they'll adjust accordingly.
I always chuckle when reading in the strat forums and the douchebag writing the post says "Donkey1 in SB" "Donkey2 in MP", then they go on to describe a hand where the "Donkey"'s played fine and they themselves made these stupid ignorant moves that of course didn't work, and now they're posting asking why.
Here's a few shining examples:
Example 1
Example 2