If I'm correct to suspect that this train of thought might exist (and not only keeping UI designers away, but all sorts of related professions like product managers, marketing, etc.) then maybe the whole notion of "[product] is open source, contribute to the source code or issue queue" needs to be rebranded as "[product] is an open collective work, contribute in any possible way" with a collaborationware workflow that doesn't consider changing the source and filing issues (bugs, feature requests) to be the only first class citizens.

Many projects do have communities with robust organization of leadership, management, branding, etc. but with real or perceived barriers to entry that don't give the same sense of openness and "everyone can contribute to this well-oiled contribution intake machine" in non-code aspects akin to what developers enjoy.