The purple / orange theme is gross looking and clashes with neutral colors from third parties. It just looks extremely unprofessional. They should just ship vanilla GNOME, it’s beautiful.
I'm not a designer so it is difficult to point out exactly how it should be improved. My reaction to it however is it has a very amateur look in my opinion. I've typically installed Gnome extensions to make it look better (though I think the one I've been using is probably going a little too far for most users).