If by Claus you mean Masked Man version, then yeah he's important... to Mother 3.
Realistically, Porky would be more representative of the series (being in two of them) than Claus. That would seem unfair to fans of Mother 2. None of the other characters, save Porky and Giygas.
Giygas would never make sense. Porky would be stupid in anything other than his spider-mech thing.
Other than Claus, Kumatora had her moves stolen from her. Most of the characters simply aren't as important.
Similarly, METROID only has Samus, Kraid, Ridley, Dark Samus and Weavel (for being a Space Pirate, which is core to the series). I doubt Weavel will ever get in. I'm the only person who likes Kraid, Dark Samus is possible. Ridley was made a boss because it just makes more sense.
Also, other than Mario, most series only got one or no Assists.
And if you want, you can count Ultimate Chimera as an Assist of sorts?
This game got a LOT of MOTHER representation, all things considered.