Yes! Most Open Source projects were started by coders. Many coders (certainly not all, but many) are either bad at, or bored by, the stuff you've mentioned.
Documentation is often the Achilles heel of Open Source software, so having that ability (specifically "explaining new technology to customers" and "simplifying technical issues for customers") is incredibly valuable.
UI is also often weak and bolted on over time, to a degree almost never seen in well-funded proprietary software. So having wireframe/mockup sessions with OSS projects to sort out their UI weaknesses and maybe consolidate UI elements and make it more coherent to non-technical users would be of immeasurable value.
I say all of these things with the experience provided by the Open Source projects I work on (Webmin/Virtualmin/Cloudmin/Usermin), which have about a million users...but, our UI kinda stinks in a lot of ways especially because it has accreted rather than been designed, our documentation always trails the project (less so for Virtualmin and Cloudmin since we have some money for these projects as they are also commercial, but still a problem), and we never have nearly enough hands to help resolve those problems.
While there are Open Source projects that are insular and closed off (and many that don't want to be might look like they are, just due to not having anyone welcoming and marshaling volunteers, but they probably still want help), most will welcome your input as long as it is well-meaning and exhibits some desire to learn and teach. If you spend a week with almost any Open Source project and don't see areas that need some attention in the areas you've mentioned, I'd be surprised. And, spending a week (or so) with a project is probably the minimum you need to get an idea about the culture and to know where your input would be welcomed.
In short: Do it. There is need for anyone willing to help on most big and small Open Source projects. You'll learn a lot, get a great addition to your resume, and you'll probably make some new weird friends. Most OSS projects won't beg for help, even if they need it desperately...but, it doesn't mean they won't be happy to see you. They're making Open Source software because they want people to use it (usually), so, when you use it and especially when you help make it better, you're doing them a favor, and most will take it that way, even if you aren't super productive at the start.