That is complicated. I see why they are struggling to get donations. UI/UX matters.
A perennial issue for GIMP, at least their branding is consistent though :)
User experience is the study in how to optimize an users' experience, so that they can solve issues or tasks faster, easier or better. UX might affect how an app behaves or how it looks but it isn't tightly tied into the visuals of an application. Aesthetics are just a small part of UX but usually a rather big part in UI.