One or two matchups isn't indicative of an overall matchup spread. Those characters were not relevant at all during Brawl's final days. Pikachu had a niche matchup against Meta Knight, which was arguable in the first place, and greatly debatable, but have you looked at his Ice Climbers matchup? Invalidated. Young Link, for example, has good matchups against Peach and Jigglypuff in Melee, but is he a high tier? No, he is barely middle tier.
All those characters I mentioned, are either:
1. Meta Knight
2. The closest we can find to an even matchup, and what the community proposed as a measure to deal with Meta Knight, other than dittos; Ice Climbers.
3. Characters that are very good, with solid players, who do well against Meta Knight, and Ice Climbers, respectively, and have won supermajors. The two exceptions here are Marth, who did decently against every character, but struggled against Meta Knight. Still, Leon and Mr. R used Marth in Brawl and were very good, and did implement counter-play to deal with a constant, bad, re occurring matchup: Pivot Grab > Grab Release shenanigans. And they won supermajors.
And also, Falco, who struggled against Ice Climbers, but DEHF won supermajors, and again, like Marth, had one bad matchup, and that ONE bad matchup, makes them gatekeepers, it makes them BORDERLINE viable. ESAM never won supermajors with Pikachu, although he had extremely respectable placings. That is how crazy it was in Brawl. 37 characters, and if you were bad against the top 2? 3? 4? You were literal trash.
Diddy Kong as good as Meta Knight, also? That's pretty questionable. Very questionable.