All of the mounts from reputable manufacturers thatI’ve looked at are either rubber or Velcro or have a quick release that’s designed to tear free on impact.
Paracord is incredibly strong - some have a tensile strength of over 200 kg /550 pounds.
The concern is that your head might hit an object (such as a low tree branch) and the light or mount will snag on it, whipping your head and neck back. The neck vertebrae don’t like this motion.
Also, in an accident the helmet is designed to slide along the ground preventing torque to the head. Your paracord might also snag and cause the helmet to torque your head.
Note it would take a freak accident for this type of injury to occur. An individual might be fine with accepting this type of risk personally. A manufacturer who has domestic assets that could get seized in a lawsuit might not be willing to accept the risk, thus they err on the side of caution. On this forum, we can’t say “it’s fine” — that’d be negligent but we could say that many people would find the risk acceptable but ultimately it’s your call.
Tl;dr: ideally, you’d want your mount to have a full break-away design.