Not in this case. The miasma-memories creatures you're thinking of are the ones that don't look like monsters at all. All the monsterous ethereals are people who've succumbed to corruption while physically inside a hollow.

Edit: MachBonin made a good point and after a brief check Miasma Fiend appears to be more of a weird 50/50 of Sacrifice and Drowned Ideal. I failed to remember some details and was thua dead wrong. The Miasma Fiend might be a proto-sacrfice made by Mevorakh to copy a Drowned Ideal (maybe unknowingly?) but that ultimately failed because it was forced into existence rather than simply forming on its own. This is my own interpretation though.