I know, not a high bar to pass. One might even say it's below sea level.
Mimics are Aberrations, I'm not sure darwinism applies to them, especially since they reproduce by splitting like oozes and are widely accepted to be either created by wizard or imported from the Far Realms. But also, you have to consider the one argument that can justify the appearance of basically any trait : "it started as a courtship ritual", because transforming into the craziest thing is simply really cool, or, most probably, simple camouflage, since even their natural form resembles a mountain rock, down to the rock-hard skin. Those are not applicable to kelp anglers since, well, SLAs have no visual manifestation.
I don't know, you try punching moss/an unrooted bush and tell me if it broke.
Oh yes ! We need a D&D Bone War ! Like somebody found the skeleton of a Ha-Naga surrounded by the limbs of its victims and thinks it was some sort of creepy centipede-like thing, or an adventurer destroys a Skirr, thinks it was from a single creature, and describes something with a theropod body, bat wings and a bull head. Then another erudite points out that the "wings" should have been on the feet and that it only had ultra-large webbed feet to run on water and charge to impale preys on its horns.
True. Sometimes you just want to pet a Paraceratheriida. But sometimes you want to pet them all !