My experience with trying to help open-source projects with design has been quite bad. Projects that most need design help are usually not sponsored by a company, a true contributor project. Design is opinionated, and for many projects it can be hard to find consensus. Much easier to find consensus around adding features.

Two very different examples: Blender and Audacity (pre-acquisition). Blender was able to pull off something spectacular with 2.8, a full rethink of UI, controls, even app structure. But Blender is basically a company, with full-time employees, it's just that their output is open-source. Audacity, as far as I know, didn't have full-time employees. And as it grew in features, it never had any significant UI cleanup. Menus just grew, icons were tacked on. It might change now that they have been acquired.