Goblin makes game UI art, so this is from the making side rather than a player survey.
Pick one pixel size and keep it. Mixed pixel sizes ("mixels") are usually the first thing pixel art fans spot: a 1x world with a 2x menu, or pixel icons scaled by 1.5. Draw the UI on the same grid as the game, then scale the whole screen by a whole number (2x, 3x, 4x). In Godot 4 that's Stretch Mode "viewport" plus Scale Mode "integer" under Project Settings > Display > Window.
High-res UI can work too, but make it a clear choice. Smooth text and clean panels over a pixel world can read like a modern frame around a retro picture. It looks off when the high-res layer uses pixel-style art at a different pixel size.
Text: use a real pixel font at its native size or an exact multiple, and check it at the smallest window you support. Things people read fast (prices, damage, button prompts) get a 1px dark outline or a solid backing box.
Icons: choose one base size (16x16 and 24x24 are common) and keep every icon on it. Make them solid shapes. If two silhouettes look alike at a glance, they'll get mixed up.
Palette: let the UI borrow the game's palette, plus one or two accent colours saved for "you can click this" and "careful". It stays in the world and still reads.