Hydrogen peroxide is used to clean an abscess. It's unclear if this cat has an abscess or a hematoma, so a vet is best for the correct course of action to treat the cat properly.
Not anymore
"Use of hydrogen peroxide to flush abscesses. ... Due to its weak disinfectant and antiseptic properties, hydrogen peroxide as a 3% solution was often used to treat cat fight abscesses. However, it has fallen out of favour because it slows healing by affecting tissue growth through several possible factors."