mp3 is a compressed lossy format and not ever meant for audio ‘work’. Its uses are to send audio across the internet with smaller file sizes keeping audio quality perception . It is not high enough quality as a basis for audio production, generally.
I understood your point of view. However, previous versions of Cubase allowed working with mp3, and now bo Cubase 12 does not allow. I even try losses, I think the user could define which format he wants to work with.
Sure, nobody doubted that, it’s just that they get converted to wav on import, and the OP would prefer to have smaller project sizes for live use.
One solution, as I already wrote, is to use FLAC, which does not get converted to wav (you can actually set FLAC as the format for recording audio in the project setup). Not as compact as mp3, but less file size than wav.
The online documentation also states that compressed files will get converted upon import:
If you chose a compressed audio file other than FLAC, Cubase copies the original compressed file and converts it to wave format (Windows) or AIFF format (macOS).