Thanks for the quick reply!
I'm only using the in-game "toggle fullscreen" option and it starts in windowed mode, looking like this:
It is crisper, but with ~3x4 pixels per letter, it's still a bit hard to read, compared to the other text in the itch.io app, for example. Switching to fullscreen in-game shows the other screenshot I posted (Alt+Enter has the same effect), scaling it up with a factor of about 2.4.
Not really sure what's causing it either, but if you managed to find and fix it, that would be awesome. Tried to play it in windowed mode with the Windows magnifier at 500%, which worked okayish, but still was a bit blurry in some places, most noticeable with the prices and texts in the shop/upgrade menu before warping (even with the magnifier smoothing option turned off).