Gimping Sonic with Mario is difficult seeing as the cape gives him 2 springs and Fludd doesn't do much unless he's in the tumbling animation after spring. When trying to gimp him, you're reduced to primarily using close ranged attacks like reverse Uairs and Bairs to try and score your kills. Sonic is just in that small handful of Hard-to-Gimp characters with G&W and MK for Mario.
Mario has defensive options against Sonic's edgeguarding as well, however, because fireballs hinder HA and Fair before they hit, upB goes through spring, and upB stage spikes when you don't aim your Bair stage spike effectively.
If you're looking for a list, alone Fludd gimps: CF, Ganon, Falco, Fox, Ness, Link and all characters that can't sweetspot instantly from upB (Wario, Ike, Kirby). This is a tiny group compared to the cast, but that's because Fludd is never used alone, it's always used in conjunction with another attack because all it does is push and slow momentum. It's not one of those attacks with a clear-cut purpose either, it just adds to Mario's versatility. Fludd, for example, cancels out Luigi's and Pika's SideB and actually reduces his motion completely, rendering these attacks uneffective. Fludd also pushes Snake's grenades back to him when thrown without causing them to detonate, and kills loose Pikmin that aren't following Olimar (like after a Smash) if they're not blue. It can help recovery to an extent as well.
Even if you take away Cape and Fludd, Mario still has options like fireballs to remove second jumps, reverse uairs offstage, Bair stage spikes and Bairs away from stage. His Nair also has good priority, and cancels out attacks like Diddy's upB.
Also, I wouldn't go that far Judge. It's still a 2:8 matchup.