I tend to think the world is a bit of a miserable place, so anyone who can add to people's optimistic, cheerful side is doing a good job, which is what I hope I'm doing.

This quote is saying that the speaker believes the world can be a negative or depressing place to live in. They think that people who can bring positivity and joy to other people are doing something good and helpful. The speaker hopes that they are one of those people who can make a positive change in the world.