We actually have a design now specifically to track user's UX/UI feedback, complaints, and suggestions: https://gitlab.gnome.org/Teams/GIMP/Design/gimp-ux/-/work_items
We also highlight UX/UI work done in our news updates, to encourage more designers to get involved. But I'll admit it's an uphill climb to convince people that we care about this. I think we're making progress though!