A lot of open source software sorely needs help in the UX/UI domain, though there is a pretty major obstacle when trying to contribute time in that domain: Each UI/UX issue can be either a lot of work to resolve correctly or it can be simple enough that bikeshedding can occur when bringing up the issue.
Most of my OSS work is in the domain of Linux audio apps, which have a much greater than average (IMO) UI/UX space that they need to explore to work well. When I've had more time to work on the projects then UI/UX contributions were typically seen as a great thing to help direct where effort is spent. When I'm strapped for time they seem to highlight issues that I've been vaguely aware of, but can't address. In both cases the feedback is helpful, though the priorities shift around based upon the number of active contributors and their respective domains.
In the later case it's frustrating as a dev since flaws are highlighted and frustrating to the designer as their ideas sit without being implemented. So, if you can find an active project looking for feedback in the UI/UX area, then you can have some good luck, otherwise many projects can use a hand in building graphical assets for interfaces (and that's not a skill many normal devs have).