Just talked to a friend of mine that is in the "business".
He told me that even today, on primary radar, wooden aircraft can be tricky to pick up.
The signal is worse. Same goes to fiberglass. Yet, he sometimes picks up birds, or rather flocks of them.
Now, the radar today is very much better than in WW2. So, I'd put some pennies on that when the Mossie entered operational service, it did cause problems for both the speed and the poor signal. It would also make it more problematic to estimate the strength of the force.
No AH dot-dar in WW2 you see,,,,,,