I'd agree he's a little underrated, as I sometimes see him placed bottom 5, which I feel is too harsh.
I think he really lives up to his title of "the king of swing" in Ultimate. He's fast, can kill mad early, and has great range. He can win matchups he has no business winning because his advantage, weight that means he often takes advantage of rage, and Giant Punch are absurdly good.
The problem is, his strengths are offset by extremely glaring weaknesses. He might have the most trouble escaping disadvantage in the entire cast, lacking a projectile, a teleport, multiple jumps, counter, armored/at least good hitbox for hitting from below on nair, a dair that can outrange things or hell, even a stall-and-fall dair. This is compounded with the fact that he's a huge target, and is often forced to rely on his airdodge, bad though it may be. Every other character has something, usually multiple somethings, to help in this regard, which speaks volumes to how bad of a drawback this is for DK. And what makes this worse, is his recovery and options for escaping the ledge are also very poor, making him among the few characters you want to try to edgeguard (And I personally find myself very frustrated now and again, at how great an edgeguarder DK could be if they had just given his recovery a little more vertical distance. Dying because I tried to take advantage of what should be great offstage tools while not even being that deep offstage feels bad, man). Aerials and projectiles being so free in Ultimate also means he doesn't get to take advantage of his grab game as much as he'd like. Oh yeah, projectiles, he sadly doesn't have an answer to them and lacks one of his own. And oh yeah, shield pressure... he has nair and up B as OOS options, but they're poor OOS options compared to the vast majority of other characters.
Sadly, all of this bogs the ape down, and it's a bit of a shame his weaknesses weren't mitigated more through buffs. I wouldn't want him to be a menace, mind, but he could have been a high tier threat still kept in check by the weaknesses you can't lessen, such as his size. As is, at least he's so fun that ya can't help but love playing him in spite of everything!