FWIW DK has been seen as at least a decent character in three straight Smash games (Melee, Brawl, 4); It's just that in Melee and Brawl "decent" = "very rarely seen in Serious Business settings". The high point is no doubt Smash 4, specifically after a patch introduced the "
Ding Dong" kill confirm. It got so notorious that
not only did Ultimate go out of the way to nerf it specifically, they also made grabbing riskier for DK even if the reward was overall improved.
DK currently is in kind of a rough position, so yeah, I can see changes there. Very easy to pick up and play - and he works fine as a casual character. And he's not considered a bad character However, his most pressing practical problem is that once he's on the backfoot... oh boy. One could make the argument that it's a problem casually - he has little defense if something clips his giant frame - and in competitive scenarios due to the above. He does have armor... but on three special moves, which is more situational than Bowser's and K Rool's.
As far as canonical representation goes, yeah no complaints there. The main issue - one which I'm assuredly not alone in holding - is that he currently feels like he's just doing generic gorilla thing. Which was fine considering 64's short development time, but isn't the direction conductive to canonical representation to begin with.
I wouldn't mind the Gun - if you ask me,
replace Headbutt. Giant Punch is the prototypical big hit and a very casual-friendly attack, there's certainly an argument for keeping it intact. Headbutt doesn't have the same luxury: it's been since Melee a really bad example of "generic gorilla thing" and a situational move rolled into one. You know what to expect whenever a DK player uses it (especially with a charged Giant Punch). The only interesting thing I've seen it used is to use its armor to help snap ledge. And even then both Giant Punch and Spinning Kong have armor, so it has competition in that department too.
There's also a case for solving both a practical and canonical problem for DK - barrels could improve his disadvantage problem while simultaneously improving his connection to DK canon. Up B might be a really goos slot: Spinning Kong's been consistently improved (and is a good-to-great out of shield counterattack) but it's not been amazing, whether casually or in Serious Business hands.
(Also, I don't really understand why DK has four moves that meteor / spike, feels like overkill.)