The app icon needs to be provided in OS-specific formats:

• .ico for Windows
• .icns for macOS
• .png for Linux

compose.desktop {
    application {
        nativeDistributions {
            macOS {
                iconFile.set(project.file("icon.icns"))
            }
            windows {
                iconFile.set(project.file("icon.ico"))
            }
            linux {
                iconFile.set(project.file("icon.png"))
            }
        }
    }
}

See more from the official compose multiplatform docs: https://github.com/JetBrains/compose-multiplatform/tree/master/tutorials/Native_distributions_and_local_execution