Recently I needed to do some ad-hoc drawings and I found myself using one of these online paint clones. They are not as comfy as desktop apps but they are doing the job good enough.
I would really like to see Paint.NET running on macOS. I think MS efforts for making .NET cross platforms are somehow half-assed. They ported just so much to run web apps and REST services, but totally gave up on GUI stuff. Maybe it's their strategy to lock up devs to windows machines, as there are many tools like ILSpy that .NET devs often use.
The community answer, the last time I checked, was AvaloniaUI (https://github.com/AvaloniaUI/Avalonia) which is cross platform port of WPF (at least XAML based). ILSpy is already ported to it (https://github.com/icsharpcode/AvaloniaILSpy) but it runs poorly on Linux & mac (I have experienced random crashes and other UI problems). I hope the project will do well and within a year or two we will also see a port of Paint.NET to it.