For how relatively well known it is (it's probably like the next most well known piece of FOSS after Linux and Blender) I can't believe how bad a piece of software GIMP is.
I hope you mean the UX. I think attacking it's functionality would be unfair. It does everything good and right .... technically.
If the UX is objectively bad or "just" subjectively might be hard to find out. I would assume if there are objective UX mistakes, some contributor might have been able to deal with that by now. But of course it doesn't change anything if a majority doesn't like it for subjectice reasons. It's part of UX design to deal with subjective aspects.
Or color spasecs other than sRGB (8 bits/channel). I've a camera that takes 10 bits/channel photos, a monitor that displays 10 bits/channel, etc. But GIMP will just distort the colors because they hard-coded the color space! Can't edit for print either, no CMYK. GIMP is an image editor for the noughties, not the 2020s.
Then again, we're talking about MS Paint here. If Paint fills your needs, GIMP will be fine.
Not a professional either, but I was also curious and learned:
It's a layer of which the properties/filters apply to all layers below. So you can basically try around and manipulate the visible image without having to combine the layers first.
I've been waiting for years for "non-destructive edition" (AKA smart objects). It's a fundamental feature that I use (almost?) always as a first step. IMHO a lof of professional work is not practical without it.
They had it on the roadmap (see 2020 archive) for years marked as "No[t started]". The current roadmap looks more promising with "link layers" marked as WIP and saying it could be available on GIMP 3.0.2.
Paint is not designed to be GIMP/Photoshop alternative. It is just a simple drawing program. Although it is great that they have finally added these long-awaited features, as I may finally move from paint.net, which is also great but it has one huge drawback - it is not a single window, which is a hassle.