I'd say they should be equally important.
Come for the characters, stay for the gameplay.
I'm sure most people first played Smash with the characters in mind, not the gameplay, which is what kept em coming back. This should be evident from the fact Smash was once Dragon King: The Fighting Game. Nintendo characters helped provide the game atmosphere, moreso than what original characters could do. If Dragon King came to fruition, I really doubt we'd all be here on Dragonboards talking about a HD port of Dragon King 2 for the Switch, even if the gemplay was stellar.
Likewise, if Smash 64 played like, oh I don't know, PSASBR, we wouldn't be here.