u must had play a crappy Sonic sir. And no any character in thsi game is beatable. no one is no problem for anyone in thsi game. Why? Well, if DDD can fall off and die like everyone else then yeah he beatable. I would DDD has a lot good stuff over Sonic, but it still doesn't change the fact that DDD can still lose to good sonic. Get your facts right sir.
Eh, I'll agree partly.
If you slaughter someone with a character you almost never use,
3-stock even, then there's either a huge skill gap (either you're too good and he's decent, or you're decent and he's a POS) and/or lack of matchup knowledge on the loser's part. Most likely, that Sonic player was a POS and has no idea how to fight Dedede.
Anyway, campy Dedede is troublesome for Sonic, since B-air will outprioritize your aerials (including spindash jumps, homing attack, F-air, N-air, etc) unless you're under or above him, and a well 'mindgamed' Inhale (eg, camp your landing) will catch you. Sometimes B-air will trade hits with F-air, but you'll deal 2 damage and eat like 14, or whatever D3's B-air hits, so it's bad to try to hit it head on.
Pivot grab can counter any of Sonic's "committed" approaches, like dash attack, spindash roll (if you're trying to hit D3 with the roll), etc. It also keeps him facing away from you so it's easy for him to do a SH B-air if you try to jump out of the spindash.
When done correctly, his CG is inescapable, too, unless you CP a hilly level.
Dedede, like most characters, have to change their playstyle to adapt to the Sonic matchup, otherwise, they'll do crappy.
In fact, the worst in the game...Sonic just too quick. 
