It's a very niche troubleshoot but it could help others, so I post it here in case someone is looking for a solution in the future.
How to fix the Fullscreen / Resolution issue if you play on Mac OSX via Whisky
- Start Starmaker Story.exe via Whisky and wait for the main menu.
- Press "⌘ + Enter" to "unfullscreen" the game, it restarts, wait for the main menu.
- Change the resolution to 2560 x 1440 via the setting button on the main menu, it restarts.
- Profit! The game restart in fullscreen with the right ratio and it fixes all the rendering issues. (But you have to do it every time your start the game)
I dunno if it was already posted because I couldn't find the solution for mac specifically in the thread.
Also if some of you need some help to use Whisky and play on Mac feel free to ask I can make a rapid tutorial, it's really not so hard.
I'm not an english native speaker so excuse me if my choice of words are suboptimal. 