Well since I mod Skyrim, it could incompatible. There are Skyrim mods that could be incompatible with the game. Maybe that was the problem with your Princess Luigi costume, it may be incompatible with Brawl.
No it was, just the exploit used to add him in didn't work and because it went through my custom stage builder now I can't use or make any custom stages. Everything else is fine though.
Princess Luigi has only one Smash Bros mod available rn so that is why I know I have the correct one.
This advice is terrible and no one should follow it. Also, mods don't fudge around with your game disc unless you installed something on your system that didn't work and frankly, that would be mostly your fault if you didn't follow all instructions exactly.