DeDeDe is my main and I love him. I feel as though he is the most defensive character in brawl, which suits me just fine. Between his heavy weight and long(though admittedly punishable) recovery he is very hard to kill with good DI, he also has a fairly good projectile and long ranged attacks that give him a solid range advantage over many opponents. But most importantly is his grab game. Best grab game in Brawl period.
To start with his grab has great range, and since I already said that I feel he is a very defensive character sheildgrabbing pretty much comes natural to him. Then of course you have the great chaingrab, and his forward and back throws both do good damage. It was already mentioned that they won't be getting kills unless the opponent doesn't DI but they are still effective for racking damage. On stages that have walls its pretty much game over once he gets a grab. He can chain as long as he wants then end it with an Utilt for the kill, which in my experience is inescapable, at least I've not seen it escaped before.
Another important thing to mention is his swallowcide. I think alot of people underestimate just how easy this move is to land, and once you have them swallowed its pretty hard for them to break out. And even if you don't suicide its still a solid move for punishing certain actions with 10% damage.
Someone mentioned earlier that he had trouble approaching but I don't think its really all that bad. He can use Ftilt for certain characters and RAR'd Bairs against others. The only real problem is with characters with better projectiles than him, which admittedly there are a good number of. But alot of projectiles, like the blasters for example can be meat shielded with waddle dee's.
Finally, DeDeDe is great at killing. Alot of people look at his devastating Fsmash and think thats what his kills are like, strong and slow, but he has some surprisingly fast kill move. Most importantly is his Utilt, but he can also score some kills with Dtilt too if its fresh and that move has deceptively long range. He can also get kills with his dash attack at ridiculously low damage, and his beastly edgeguarding should be a pretty obvious choice for the kill. Also I think that Gordo is the only projectile left that can score kills now that Samus' charge shot is weaker.
Alltogether I think DeDeDe is a great character with alot of strengths, his only real weakness is some bad matchups against characters that can outspam him, but most of them can be taken care of with the right counterpick stage. His recovery is easy to punish but you should really be able to recover with just his multiple jumps until he gets to really high damage. And most importantly as someone has already stated, he has a good matchup against Snake. That should be enough for high tier on its own.
I don't think he has quite enough good matchups to make top but I do feel he has a place in the high tier.