This theme can be interpreted however you like – it can be part of the core game mechanic, or the story. It might be about reflecting something, copying, or making it reversed. As long as you can explain what “mirror” means to you, it should be ok.
Keep in mind there are two optional categories: Web Monetization and Decentralized, and a lot of prizes worth about $20k total: from software licenses (Phaser Editor, GitHub Pro, Aseprite, PlayCanvas, Zenva Academy, Panda 2, WebStorm, Making Money with HTML5, Coil), through cash (USD), crypto (NEAR, FIL), to hardware (Ledger Nano S).
HTML5 Game Developer, Enclave Games indie studio founder, js13kGames competition creator and Gamedev.js Weekly newsletter publisher. Mozilla Tech Speaker, Gamedev.js community firestarter. Organizes meetups / workshops / hackathons in Poland, passionate about new, Open Web technologies, excited about WebXR and PWAs. Likes eating sushi and playing Neuroshima Hex.