I think the music that plays after getting a Perfect/Superb (on the picture after each game that changes to reflect your score) might work better. Have him blush or smirk at the camera during this as well. Maybe with a 3 line analysis of your game could be on the victory screen (Your smashes were great, bro! Your item usage was resourceful and clever! Your stage recoveries were breathtaking! PERFECT!).
EDIT
My idea for Karate Joe's moveset
Special: Hit 3. Objects fly from the screen towards KJ; he can hit them with jabs etc to send them flying at enemies as projectiles. You can hit all 3 or just some of them if you want.
Down Special: Barrel-Bomb. Similar to above, but the Barrel is harder to break, while the bomb deals more damage.
Side Special: Punchpunchpunchpapow!: Basically his RH Wii combo, you can choose how far to go with it.
Up Special: Not quite sure. I don't want to pick something from a specific RH Rhythm game that Joe wasn't in, so either a series-wide reference or something original. Maybe he's sent flying up in a barrel, which he breaks out of by attacking.
Main gimmick: All of Karate Joe's smashes and tilts have multiple parts, like MetaKnight's F-tilt, Link's F-Smash. You can wait up to a beat and a half before the move ends, so you can attack with each part in succession, after a pause etc. Combined with the ability to miss hits from other attacks, and Karate Joe has a lot of variance in how he can attack which can throw people off. Possibly even other attacks like his aerials or dash attack could be multi-hit.
Weight would be between medium and heavy; fast fall speed. His main strength would be his unpredictability, since all his attacks have some variance.
That might seem like much, but Robin has aerial smash attacks after all...