Unfortunately, sideloadly does not support keymapping plist.
If the game has the option to support the controller, run the game, go to the Game app's Preferences at the top of macOS, and turn on Controller Emulation in Game Controls.
Keyboard or mouse operation is possible with the specified key mapping.
However, customization is not possible like Playcover.
Even if you use Playcover to inject a playmap file, it does not work on Sideloadly because there is no keymapping-related dylib.
If you have time, please try the following method.
First, set the key mapping of the desired game on Playcover and save it.
Download IPA by exporting Sideloadly from the File menu of Playcover.
Install the downloaded IPA on macOS using Sideloadly.
According to the PlayCover manual, when IPA is exported, key mapping information is also stored.
I didn't try it myself, but there is a possibility.
Good luck!!