I'm not involved with Gimp development, I've been watching it from the side, so I can't tell if there's an actual lack of contributions related to UX design -but so far I have only seen the public respond with the same sort of vague feedback : "the UI needs work". Unfortunately that's as unhelpful as it gets. Spending some time designing interface mockups, or writing up descriptions of how such and such feature should work, now that's helpful, and is something pretty much any user can do.

I was making a general statement about why foss stuff doesn’t tend to suit glitzy, highly marketable front facing stuff, using gimp as an example

Yea, I believe that's true. And it is always a resource problem, because with limited resources, developers focus on making the thing work first, look nice second