"It's fun to read manga."
If I recall correctly what my Japanese teacher said years ago, I believe you use の to refer to something you've experienced or that's in some way familiar to you. Using こと would be a little weird here, because if you're saying something is fun, that implies that it's something you do. I guess you could possibly say 漫画を読むことはたのしいですか to ask someone if it is fun if you've never read manga and have no idea what it's like.
I lived in Japan and found こと to be more common as a normalizer /gerund creator because no is used as a particle