To say something on a funny note means to say something using humor. This is usually in contrast to something serious that has been said before. The contrast can be between the tone of the remarks (funny or serious), and does not necessarily mean a change of topics.

when changing the topic to something more humorous than the original topic

Sarah, did you mean that I forgot to write the word "use" after "we" ?

This doesn't make sense. :( Do you have more information that you can give?