The Shadow Mantle's mouth has the same color as one of the tones it uses for the fire attacks it does.
Could be that the "mouth" is just a fire holding the Mantle up. Or that the mouth literally is just full of fire inside.

>The "mouth" in the Shadow Mantle fight matches one of the secondary fire colors it uses in attacks
New schizo theory - I think this thing may literally just be a little bit of fire or something that is controlling the Mantle.