Ugliness is something I found to be quite common in Java based apps. I never understood why, though.
There was a talk [1] at Devoxx 2023 by an Oracle which gave an overview of the current state of the UI ecosystem, and it wasn't too bad, though it seems like it's mainly JavaFX these days. Support for fractional zooming on HiDPI screens on Linux is still shit, unfortunately, which is something that drives me nuts.
What .NET GUI libraries are you referring to because anything other than WPF is tetering on "not a good idea." Have you built anything in Swing, WPF, Winforms, Xamarin Forms, or MAUI? You should give them all a serious test drive and get back to us with some informed comments.