Thanks for your input!
Diving deeper into this, I got intrigued by possible SVG supprt – real vector-scaling would be neat.
So I experimented, imported the same graphics both as PNG and SVG and set them side to site on display, both in those three scale-versions (original, 0.5 and 2.0). Then re-imported in all kinds of Filter/Mipmaps on/off combinations and compared both on monitor- as well as on mobile-screen.
With 1080x1920px the overall project-resolution generally is bigger than both displays – it's upright format, so not "turned 90°" on PC-monitor and the old phone isn't "Full HD".
However, my Test Width and – Height is exactly 50% of the project-resolution, which appears to lead to some smoothing and general "better looking" on display. With Test Resolution turned off I can obviously only see half of the project, but the test-results come "unconcealed". That's how I chose to test (while the automatic down-scaling on the phone-display always looks a bit better in that regard – I'm checking with a magnifying glass... ; ).
One thing is clear from the start: SVG doesn't import gradients, only one-colored fills, leaving any gradient-filled plane completely transparent.
- Both Filter and Mipmaps off looks rather bad in all versions.
- Mipmaps only on: no obvious change, still bad (although the non-scaled versions appear nice on the phone!).
- Filter only on: much better for original- and upscaled-size, still bad for downscaled.
- Both turned on: looking rather good in all versions, especially on the phone. On the 100% PC-screen there's an ever so slight blurriness in all versions though. I think the amount of blurriness is about the same in the upscaled and the downscaled versions, but it appears stronger on the small one, probably because, well, because it's smaller...
I couldn't make out any differences between PNG and SVG though.
(But I hear that graphics-import will be reworked in Godot 4.0, did I get this right?)
I think depending on the graphics to import it's always good to test case by case, but overall turning on both Filter and Mipmap seems like the way to go. Too bad SVG doesn't seem to offer any advantages (yet?).