No way is it in Pikachu's favor. Mario can do so many things to Pikachu with that cape of his. He can reflect Thunderjolt and use Thunder against him. He beats Pikachu offstage. FLUDD absolutely wrecks Quick attack, as does cape. Pikachu cannot edgeguard with Thunder, because Mario has the Cape. A good Mario will NOT get grabbed, the cape can turn around anyone that tries to grab him. If both of them are offstage, Mario can FLUDD Quick attack and send Pikachu falling, while he uses FLUDD to recover back to the stage.
I dont really like matchup numbers, but I say 65-35 Mario, maybe even 70-30 Mario.
Just hope you're a lot better than whoever's using Mario.