Dazzle isn't meant to hide. Dazzle is meant to confuse a human trying to estimate course and speed in order to send an unguided torpedo at it. You need to look at hit rates to judge effectiveness.
We don't have that data. All we have is conversation from former captains saying it didn't work.
EDIT: Also, IIRC, the most successful captains in the field didn't actually use the AOB method of determining the target's course. They would instead cross in front of the ship at a distance. When the ship's masts lined up they knew the ship was pointed directly at their current position and could calculate from their current bearing and the bearing to the target exactly what the target's course was.
In hindsight what the Brits should have done instead of odd paint jobs is offset their masts by a few degrees.
