looking in the MOT manual there is not a lot mentioned about mudguards except for this
Some fork arrangements rely on the bracing incorporated in the mudguard fixings to maintain their alignment. A mudguard insecurely fixed to the forks may therefore adversely affect the handling of the machine
Check the security of the front mudguard and look for evidence of it having been in contact with either the wheel, the tyre or any fixed part of the machine.
and in the failure section
An insecure mudguard or one able to easily contact the wheel, tyre or any fixed point of the machine.