What the character of Fowler means in Graham Greene's The Quiet American when he says the phrase
Innocence is like a dumb leper who has lost his bell, wandering the world, meaning no harm
is basically that we give too much importance to the concept of innocence. Innocence to many equals dignity and purity. To others it means the lack of capacity to do harm, or the embodiment of good will towards others.
Fowler disagrees with this. In his view, innocence is neither good nor bad: It is simply a trait that some people are born with, while others adapt due to weakness. However, in Fowler's view, a person would never opt to become innocent, or use innocence as a personal strength. Why? Because iinnocence does nothing. It does not make you greater, nor more prone to salvation, nor does it protect you or enlighten you. It is, in fact, a way of life that may render you prone to more danger.
In all, innocence is not a gold medal one shows around to brag about. Fowler feels that it is a bad choice of mental state that does not make you a better nor worse person. It simply "is", and is not deserving of any admiration.