OK, I didn't consider that the Mechagon mobs are mechanical. But in Nazjatar most mobs are "mortals". The quest doesn't specify that they have to be humanoid, but I guess lore-wise it'd make sense. Even then, Nagas are humanoids, and those lobsters, too. I'll just accept that I'm unlucky, not like it matters in the long run if I get the transmog a couple days earlier or later.
[edit] For what it's worth, just tried it on another alt and got 5 echos after killing 20 mobs.