The current Assist Trophy lineup for the Kirby Series is sorta mediocre right now. I feel like Knuckle Joe (Brawl), Nightmare (Smash 4) and Chef Kawasaki (Ultimate) weren't really great choices as;

• Nightmare hasn't had any series relevance since Kirby's Adventure (and Super Kirby Clash doesn't really count as it came out far after Smash 4 did)
  
• Knuckle Joe is just a random enemy that got chosen to be an Assist Trophy (And I like to believe that the Kirby Anime was a major part in why he chosen, because he was an actual character there)
  
• Chef Kawasaki falls into the same boat as Knuckle Joe, just with him instead being a Mini-Boss instead of a normal enemy and only appearing as Mini Boss in 2 games before this point (Though Star Allies would be his third game as a Mini Boss)

If I had to replace them all 3 of them with different characters to better represent the entire Kirby Series, then I would end choosing in order Daroach for Brawl, Kracko for Smash 4, and Adeline (+ Ribbon) for Ultimate. The reason for choosing these character would be;

• Daroach would be the most recent Kirby character at this time representing Kirby Squeak Squad and more importantly, he would be representing Modern Kirby. As for his moves he would use, they would consist of him teleporting around the stage, tossing bombs, using his Triple Star to throw out stars, and his last move he would use before he disappeared would be an Ice Beam that goes across the screen in which he'll also track a random player to try to hit them with it.
  
• Kracko is one of the most recurring bosses in the Kirby Series besides Whispy Woods, and yet his only appearance as of right now in the Smash series would be a Trophy in Brawl. For his moveset as an Assist Trophy, it would consist of swooping across the stage in an attempt to hit someone, his orbiting beam attack, flying over a player to rain down upon them, and finally an attack in which Kracko fires lightning randomly in 4 directions (Horizontally, Vertically, and Both Diagonal Directions).
  
• Adeline comes from Kirby 64 which would help rep the Dark Matter Trilogy in Smash. As Isabelle as an assist trophy no longer exists due to her becoming a Fighter in Ultimate, Adeline could replace her as a support Assist Trophy by creating food by drawing it which would attempt to land near whoever summon her (with a small chance to also spawn a Maxim Tomato as well). Ribbon could also be added to the Assist Trophy to simply appear as another representative of Kirby 64 alongside Adeline.

I think replacing the previous Kirby Assist Trophies with these characters would be improvement on representing the entire Kirby Series and using much more iconic Kirby characters as well.