Is clamped fins not a sign of stress?
Indirectly, yes. Clamped fins is due to some problem which might be a disease, inappropriate water parameters, poor water quality...whatever. And any of these is also causing stress. But the stress itself is not the cause of clamped fins, it is whatever is causing the stress. And that "whatever is causing stress" is what needs to be fixed, and if this has to do with something in the water, water changes will benefit more than not.