In general I think people put too much emphasis on the flashy stuff when judging a character. But there are other flashy characters with broken punish games who are not constantly called the best in the game by a bunch of people including top players and other prominent people.
Other characters with broken punish games don't also reversal you in disadvantage, and aren't super hard to hit in neutral.
The examples you gave, Peach and Mario, do big damage, but they have pretty clear counterplay in netural and are easy to overwhelm in disadvantage. If you have a disjoint you can anticipate most of Peach and Mario's burst options and reactively shut those options down (Peach side-b and Mario FLUDD being notably difficult to react to regardless). On the other hand, Pikachu is the one reacting to
you with Quick Attack if you try to pull the same on him.
If you hit Peach or Mario into the air or offstage, the paths to finishing their stocks are pretty clear. If you hit Pikachu offstage, you could get Quick Attacked near ledge and then suddenly you're the one in disadvantage and being carried to the blastzone. This happened to Leo at Glitch while he was ledgetrapping ESAM. Leo predicted a ledge jump and up-aired but got hit by Quick Attack and died for his trouble.
When people say Pikachu isn't the best, they're typically pointing to reasons like range, kill power, light weight -- those are really superficial observations that, in my opinion, compound misunderstandings about Pikachu.
People have this image of Pikachu as a combo fiend with amazing neutral and fearsome disadvantage tools. Two of these things are true--Pikachu's neutral is actually real solid, featuring pancaking, a great grab animation despite limited grab range, and safe, hard-to-predict aerials. Pikachu in disadvantage is similarly great and can sometimes reversal you.
Where Pika struggles? Advantage.
And there's the trick: Pika
isn't a touch-of-death character. He doesn't have a broken punish game at all.
Pikachu definitely has some routes that lead to death, but most of the time if you know the routes you can avoid the KO, and you'll find that you've actually taken less damage than you would from the average character. Pika combos do less % than even some of Incineroar's BnB combos in some situations. In fact, Pikachu is competing for game-worst in terms of combo damage for some of his routes.
Pikachu's frame 2 up-air is great for reacting to jumps, but it's not so great for actually juggling. When facing a Samus, Pikachu has to trade with the bombs to juggle Samus, and his up-air does so little that sometimes taking more damage than he's dealing. When facing a Ness, Pikachu presses any button to push advantage and can end up flying off the stage into the blastzone thanks to Ness up-air or b-air.
From some positions, Pikachu just can't follow up. Throw in a platform and up-air bridge conversions become frame-tight. Pikachu's ability to punish from a neutral win is determined by how well you react to tjolt; if you react to it poorly, Pika can get a lot off of the punish. If you space in reaction to it so that you can only get hit by certain angles of Quick Attack or the edge of Pika's f-air/b-air you might end up just resetting back to neutral in a couple of hits.
The low damage output is the problem; Pikachu's advantage state is one of the most inconsistent in application. Sure, sometimes he just obliterates your stock. But on average, most times, he doesn't, and that's why he's not leading the pack.
does all of that better than Pikachu ever can.
