For me, DDP about breaks even in terms of legacy. I think he did use his friendship with Eric Bischoff to excel. However, when he got the opportunity he ran with it and the fans of WCW at the time really got behind him as he definitely had some huge pops. He definitely did work some pretty solid and entertaining bouts with guys like Goldberg, Benoit, Savage, Bret Hart, and Raven. When it came time to bring in celebrities like Karl Malone (granted, DDP was largely responsible for this one) and Jay Leno DDP was the go-to guy for this because the other top babyfaces of the time (Sting, Goldberg, Luger) didn't have the right personalities to pull those particular stories off.
However, on the flip side again he was a bit of a late bloomer. His in-ring career didn't start until the age of 35 in 1991 and a lot of his early matches were just awful, ugly affairs that certainly did not foreshadow his success. It seemed like he improved at a slower pace than a lot of those destined for the top of the card. It was mid-98 that he started getting really hot. Won the World Title twice in April and May of 1999. However, from there he fell out of the main scene and did the Triad thing with Bigelow and Kanyon. Did a good run as a heel, but was solidly in the mid-card at that point. Then he went babyface again and when they tried the Russo and Bischoff reboot of WCW he hit the main event and feuded with Jarrett in 2000 which led into the whole Arquette debacle. After that he was basically just a mid to upper-mid card guy feuding with Mike Awesome, Kanyon, and doing the Insiders deal with Nash. Then he had one last brief flash in the main events when he feuded with Steiner during the last couple months of WCW. During those last couple of runs in the main event, though, he definitely was not as over as had been in 1998-99. So his time at the top was relatively short and was super over for even shorter.
As far as his time in the WWF, yeah he was misused. However, I don't think he would've reached main event status because i don't think a lot of the mainstream, casual WWF fans saw DDP as a legitimate contender. I mean DDP kept himself in pretty good shape, but in 2001 he was 45 and he looked it. However, I do think instead of a comedy gimmick he could've been used, with his mouth, reputation, and still decent ring skills to help build guys to that next level. I think if his character was handled better that would've been a much more achievable goal.
Then he had the neck injury and retired for awhile. Since that time we've seen some yoga tapes and some pretty so-so matches in TNA. So, yeah, not really leaning towards overrated or underrated.