The certifications aren’t graded, just pass or fail. My understanding is that you pass as long as you’ve passed the requisite algorithm challenges and fulfilled all user stories for the projects — nothing to do with documenting or commenting your code.
However, in the real world (including for future jobs), you’ll need to properly document your code, so it’s a good idea to get into the habit now. “Document” can mean different things: it could be anything from creating nicely formatted guides for using the API you created to simply writing so-called “self-documenting code”. At a minimum, make sure your code is reasonably easy to understand, give your variables and functions names that explain what they are for, and add a few comments where needed to explain why you’ve done things a certain way.