Thanks for the summon, but @
san. first alerted me to this possibility. I've used it a couple of times in tournament to good effect. The team is strong, and you can't just choose to fight Charizard instead of Ness to avoid Ness' b-throw because Charizard has a kill throw too (an asset which makes him strong in teams in general).
Haven't been following this topic, but I'll talk about the character I know best with updated impressions: I don't think that Pikachu has any bad matchups. Disjointed characters used to be a problem for me until I started to use more Quick Attack. Most disjointed characters struggle in disadvantage, and QA gets them there. Meta Knight is an exception since he's good in disadvantage, but Meta Knight doesn't have the range or approach options to contest Pikachu. Pikachu seems to go even with Megaman, Greninja, and Sheik, and it's close on a few others (Yoshi doesn't feel too one-sided, for example), but there are no characters who just win.
Among top tiers: sizable wins against Falcon, Fox, Zero Suit Samus, Rosalina, and Sonic; minor wins against Diddy, Mario, and Ness (another MU which is so close as to call even).
Reading this page... mobility is more than just walk speed. There's initial dash speed, max run speed, horizontal and vertical aerial acceleration/deceleration, jump height, and unreactable (or barely reactable) burst moves. Mobile characters aren't just good because they're fast in one way. They're good because they're fast in multiple ways. They also tend to have low-lag moves. Pikachu's entire aerial kit can be autocanceled out of a SH except for b-air, for example.
Also, burst movement is a big deal because having a burst move gives you an option in advantage, disadvantage, and neutral. Look at Diddy's Monkey Flip, Sheik's Bouncing Fish, or Pikachu's Quick Attack. These burst options are good for getting out of juggles, good for recovering, good for approaching, and good for following up on advantage (and in Sheik's case, good for edgeguarding). Generally speaking, burst mobility moves are 3x as useful as other moves, since most moves tend to be good in only 1 situation, maybe 2: these moves are good in 5 or 6 situations.