Well, I don't see karate Joe being as interesting, nor as good looking in the game. 3 singers is new, Karate Joe moveset... isn't, unless he took things from the others.
The implication is that he would take moves from his series, yes. Chorus Kids would have to do the same thing to be interesting.
Here's what I imagine his some of his moves would be:
(It would be cool if most of his moves took the sound effect of their origin, although that might get a touch annoying)
Jab - The punch from his first game
Jab combo - Pah Pah Punch!
Down smash - The farmer's stomp from Crop Stomp
Up smash - An upward clap matching the Clappy Trio
Side smash - Does the Wrestler's side flex
Up aerial - An uppercut that matches the pose from Night Walk
Neutral aerial - Does a 'Don don pan pan' clap
Side aerial - Whips out the Samurai's katana
Neutral Special - Break out a guitar. The shock waves hurts nearby players and diffuses projectiles when timed correctly. It could play quick soundbites from random minigames similar to that guitar game from the DS version
Up Special - A spinning roundhouse kick, maybe it could reference the spinning dogs from Board Meeting?
Side special - A charge move. The length of charge determines the type of projectile (starting with a plant pot and going up to a barrel) and how far it travels. Tapping B keeps you in place, but any amount of charge causes Joe to quickly snap to the projectile. Good for horizontal recovery.
Down special - Start speed eating like Munchy Monk. Heals you for few seconds, after which you get "Three chow chow chow!" which makes your next smash attack to loads of damage. Do you just grab a bit of healing or go for the buff? Timing is key.
Forward Throw - High, Kick, Toe!
Up Taunt - Rolls a cheer board like in Cheer Readers
Neutral taunt - does that stupid face from the end of Karate man 2
Down Taunt - Taps his foot to the ground like the Tap Dancing game in the GBA version
Final Smash - Not quite sure. Either you play a minigame and your aptitude decides the damage you do to your opponents or onions just rain down the stage,
Wow, this was way more in-depth than I was expecting.