For example, fixing typos and spellings, improving the documentation, fix minor bugs or implementing non-vital features, things like that. Are these kind of contributions worth mentioning on the resume/CV? Could I self-proclaim as "Contributor/Developer of ___(the project)"?
I would list it as "Made occasional minor contributions to <project>." I would give such a resume a slight edge in the hiring process, because it shows you find it valuable to improve things for the next person, and to contribute to an ecosystem from which you benefit. It wouldn't weigh as heavily as full-time experience though.
A lot of people want to add open source projects solely as a resume-booster, but they just choose a random project and try to do something flashy. If you chose a project because you actually use it, and are making mundane but useful contributions, that goes a lot further with me in an interview.
If nothing else, it shows that you have some experience with source control software and bug tracking. Just be wary of the fact that you could be questioned about the extent of your contributions in an interview and if you say it in a way that exaggerates the amount of work you did, people will probably be disappointed when they find out the reality.
I would suggest you only list it if it’s relevant and you don’t have a lot of development experience. If you do mention it, don’t go overboard. Calling yourself a developer on the project may be a bit much.