If you need something like
https://en.m.wikipedia.org/wiki/Pinta_(software)
use that. If you want something like krita, use that. And if you need something like gimp, paint.net is not even in consideration because it is decidedly something else
I have also worked as a “designer” in a real typography for some time and know a thing or two about the process (not fully, but count me as sort of an insider). And I can tell you that gimp is only “an artist’s impression” of a graphical toolbox, and that artist is heavily drunk. If you need something like gimp and paint.net/etc isn’t enough, you need photoshop. Cause we’re talking about serious pre-print color management, etc. Gimp is two parts: very poor home graphics editor and very poor industrial graphics manager.
Youtube thumbnails are paint.net 100%. I use that all the time for all my graphics, technical and creative, tried all others, and have no reason to switch, apart from OS requirements.
But generally it's really not as nice as Paint.net. I don't like the ui at all (washed and bleak, hamburger menus, checkboxes on layers swallow second clicks, clicks on icons sometimes demaximize window, etc, also slow). It's just bad. Idk if it's gtk3/4 or pinta itself. But it generally works and apart from that I'll give it a 4 out of 5.
Also, is "washed and bleak" really that big of a problem for an image editor? It just doesn't matter what it looks like as long as the UI is intuitive and has the features that you need. It should also be noted that Pinta very much looks just like your overall Desktop appearance on Unix. I'm on XFCE and it's so incredibly theme integrated it looks like it's part of the system.
Personally, I really like Pinta. Biggest problem is the bugs and crashes. Wish I could use actual Paint.net though, but there's no way to use it on Linux.