There are plenty of areas where we would welcome help from design-oriented contributors, be it to improve existing features or to develop new ones. If you want to give us a hand in this capacity, we recommend that you first get acquainted with OpenRefine as a user. Install OpenRefine and learn to use it by following some tutorials or watching some videos. Even better, you could use the tool on a dataset you are familiar with, to transform it and link it to other data sources, for instance.
Perhaps in that process you already encounter things that are counter-intuitive for you, workflows that could be improved, interfaces that could be made clearer, and so on. If that is the case, it is very useful if you give us this feedback. It could be on the forum, for instance in the Support category if you are not sure about something, or in the Design proposals category if you have a proposal for an alternative design. You could file a GitHub Issue directly.
If nothing catches your designer eye after using OpenRefine for a bit, then you could look at issues where design proposals are needed. Those are issues where it would be useful to have a designer propose a particular bit of user interface to address the need. You could pick one and propose something there, for instance with mock screenshots made with the tool of your choice, or describing the desired interface in any way you like. You can do so by commenting in the GitHub issue directly. If your proposal gathers consensus in the community, then this would make it easier for a developer to tackle that issue.
We also have a design team that we ping on some issues when an opinion on product design is required. You can request to join this team on the forum.