Many programs for mac provide a custom disk icon for their dmg files, e.g. Firefox or Skype. Our ISO file uses the default disk icon, which makes our ISO look less polished compared to other programs.
I didn't manage to change the disk icon on an ISO, but I could change it for a dmg file, so a swap to dmg format (bug 4329) may be needed before solving this bug.
Information about how to change the disk icon can be found here:
There are also instructions in the README file for the macOS client (added in r38730) that assumes the info about the disk icon is stored in the DS_Store file.