No. A CPL cannot be rotated into a true “off” state or made equivalent to plain glass.

Rotating a polarizer changes which polarization direction it passes or blocks. That changes how strongly it affects polarized reflections and skies, but it does not remove the filter’s basic light loss for ordinary, unpolarized light. So the transmission penalty remains even when the visible polarizing effect seems minimal.

With polarized light, rotation can align the filter so that particular polarized light is transmitted more strongly, or block it more strongly, which is why reflections or blue skies change as you turn the filter. But the filter is still a polarizer, not a neutral clear filter.

So:

• you can vary the polarizing effect by rotating it
• you cannot make it behave like a UV/clear filter
• it will still reduce light compared with no filter

If you want protection only, use a clear or UV filter instead of leaving a CPL on all the time.