I played DDD in a local tourney a couple weeks ago.
I don't think he's really underrated. I agree with above, most people feel he's just OK which is true. Pretty much smack dab in the middle of the tier list. Being slow and having a huge bit box are major weakness that bring down his other awesome attributes. He's pretty good but I don't feel he's underrated. Just underused because most people like rush characters which DDD is absolutely not. He has a playstyle that most people don't like, and frankly find boring. Also tons of people automatically write him off because they hate the gordo reflect mechanic.
Almost everyone at the tourney I was at played rush down characters...diddy, sonic, shiek, falcon, ect. And I got some hilarious comments when I was seen playing DDD. A sonic main came up to me with a very confused look after one of my matches and was like "that's your main!?!?!?!?
" He looked at me like I was nuts for playing Dedede, lol. Which I probably was because there were SO many Sonic mains there.
But yeah, to be honest he's definitely not top tier and it would be a huge achievement if someone won a major tourney with him. Playing characters like Diddy and Sonic is such an uphill battle. I personally find him a blast to play though which is why I use him.