If all fish are sentient in The Little Mermaid, Ariel wearing clamshells as a bra would be like if Princess Kate wore a human skeleton as lingerie.

Unless the clam is still alive, which is even more concerning.

EDIT: For crying out loud, everything's sentient in this movie, that's the point. That clam was a self-aware individual once; it had hopes, it had dreams! But now it's dead and its corpse is used to protect someone else's seashells.

Again, unless it's still alive. In which case, ecch.