The mallard:

• - makes the statue engaging and charming
• - links Gresley to his most famous achievement, the steam traction speed record with his loco called Mallard
• - sparks interest in, and curiosity about, the statue and the man; very few people alive today would recognise a likeness of Gresley. Even little children will want to know why the duck is there

Without the mallard, a statue of a long-dead man most people have never heard of is not going to be noticed, and will do little to raise awareness and appreciation of Sir Nigel.