For the moment, users with issues with the core downloader will have to manually download them from this exact link: Release Release - 2026-05-04 01:37:09 · MustardOS/extra · GitHub
Download the .muxzip file for what you want to install, move that file to the ARCHIVE folder on either sd1 or sd2, and inside MustardOS go to Applications>Archive Manager, select the core file and install with A.