There probably aren't that many xamarin studio users on mac and most of them would be developers anyway. If something looks out of place, developers are often oblivious to it (how long would it take you to spot a font out of place?) but artists are hyper-sensitive to it.
Re:GIMP – 2016 in review
by gurkendoktor on Wednesday January 18, 2017 @10:18PM
(#13432680)
Photoshop, Illustrator, Lightroom, and non-Adobe tools like Affinity Designer all use portable, non-native UI code. That hasn't stopped artists from using them. I don't out-of-place UI is the reason why people avoid Gimp.
Xamarin Studio is built using Gtk and actually looks nice on macOS, or at least I have seldom heard people complain about it:
https://blog.xamarin.com/the-making-of-xamarin-studio/
I think the problem is that Gimp is understaffed to produce a Photoshop killer with any toolkit.