To clear things up, Bowser Jr was in the Clown Car before Smash but it wasn't his defining gimmick in most games before Smash (and subsequently his amiibo) codified it as such. Before then his gimmick was squarely the magic paint brush (and in the Mario Strikers series, screaming).
It's not the first time it's happened either as Yoshi couldn't freely seal himself in an egg to roll and Bowser wasn't seen to retract into his shell before Melee.
Adding to that, I can list several things that influenced later Kirby games after Melee.
- Amazing Mirror had Master Hand as a mini-boss, and had a new ability that allowed you to use special moves from Smash.
- Entries from Return to Dream Land onwards gave Kirby some standard Smash mechanics in his regular moveset, namely air dodging and shield dodges.
- Triple Deluxe had a mini-game called Kirby Fighters (later getting a standalone Deluxe edition), which was basically Smash lite.
While Smash isn't all-encompassing, it doesn't just exist in its own vacuum either.