Pikachu is 70-30 or 60-40 against the rest of the cast. His closest match-ups would be Link, Donkey Kong (if allowed), and maybe Zelda.
Pikachu has everything that everyone else is lacking, hence why they are low tier. He has good kill moves (Upsmash, Nair), gimps (Upair), actual combos, actual range, speed, and, imo, the best recovery of the cast.
He can dominate Pichu, Kirby, Ness, and Young Link just because of outranging them.
He beats Mewtwo, Bowser, Yoshi, because of superior maneuverability, combos, and kill moves.
Game and Watch is killed early and is easy to edgeguard/gimp.
Roy gets ***** by grabs, gimps, and aerials.
Zelda is easy to juggle and edgeguard/gimp.
Link is easy to combo and gimp.
At least Link has even matchups and gives inferior characters a chance with his slow speed, bad recovery, and relatively bad counterpicks.
I think that Donkey Kong would balance things out with the higher portions of the cast, but I'm aware that he, like Pikachu, dominates the rest of the cast that isn't named Pikachu/Link.