Thanks for your suggestion!
Actually, I have found a way to change fonts in the Kindle app, and it's less complicated than what I assumed... Downside is, it affects the system Georgia font (which is the one used by Kindle), so no more Georgia in iBooks and other apps that use it... Here's how to proceed:
1. Jailbreak (who would have thought...)
2. Install the free app Bytafont via Cydia (respring! if it doesnt happen automatically)
3. browse for a suitable font from within, download via Cydia (you will be redirected to the appropriate package)
4. Completely restart Kindle app aaaand... done, all of your books will be displayed with the new font instead of Georgia...
The hack is reversible (and the number of new fonts is only limited to the Cydia selection), just make sure you uninstall Bytafont AFTER restoring the Georgia font...
Now if only there were actually legible fonts on Cydia... (there is a method of building your own font package and applying it but it seems less hassle-free than using Bytafont, unfortunately, the developers currently dont accept suggestions for new font packages, I will check occasionally and hope to get CMU Serif and the Tex variety of Palatino on there)...