It's a step in the right direction. It prompts people to think about something worthwhile and could easily become an ice breaker within a community. At the very least it may spark an idea in someone who reads it. "What does it really mean to 'make a difference'?" That can segue into powerful thoughts. Even the simple action of physically presenting a hope or dream can be motivating.
Of course, it's always up to the person to act but I find it's easier to work toward a goal when my spirits are high and being surrounded by hope has an uplifting effect.
I had always thought this kind of 'motivational' attempts are too simple-minded to work. Then one day, while under a lot of pressure and discouragement, I happened to see a man on the road with a T-shirt that said 'Never give up', and it really made a difference in my mindset.
So, I now believe that tiny things like this can mean a lot to someone who's on the edge and can turn them around.
I second this. I think art has the effect of starting conversations. If it makes people talk and think about things in a different way, then, you've started a process of something (potentially) greater.
Yes, something like this is easy for people (easier than doing something perhaps). They'll participate because it's there and there's no commitment (you just write things on the wall). But, what of the people who pass by it and, as Dysiode has said, what if it sparks an idea in them?
It may not directly change the world but it sure can plant the seeds that do.
Of course, it's always up to the person to act but I find it's easier to work toward a goal when my spirits are high and being surrounded by hope has an uplifting effect.