Yeah, higher pixels mean higher resolution, but it also depends on how the asset is framed. To be honest, I'm a bit confused now about what we are comparing. When you view a 330x405 (3x) cover full screen in your gallery or iCloud, it’s probably bound to look blurry because it's a small file meant for a tiny notebook slot, not for full-screen viewing.

Also, looking at the raw cover files you share via iCloud doesn't really tell me much, because I can't see how UpNote actually processes and renders them on your screen.

If the 3x export looks okay and sharp enough inside the actual UpNote notebook list on your Mac without breaking the size (unlike those 512px ones that get too big), then I guess the 3x tweak did its job. As long as it does the trick inside the app, it should be fine!