I suppose the most comparable case to K. Rool here would be Kirby. Well, he doesn't get actual personas like K. Rool does, but he's still in a similar position when it comes to costumes at least. In his games Kirby can't freely change his abilities and needs to get a certain power to use each of them which by the way give him a unique hat... whereas in Smash Bros. Kirby can use three visually distinct of these powers without having any hat, and even then he can still use them all when he gets one by inhaling an opponent. (Or four copy abilities at once without the possibility to get any hat at all if you use Ice Breath custom)
I'm not saying your point is moot, actually I'd say it may be the biggest hurdle K. Rool could have to face, the guy who made this game was the one who judged Mario's doctor persona worthy of being his own separate character after all, but I wouldn't say it couldn't be worked on. I would say even though he never used it outside of his Kaptain persona as of now, King K. Rool using the blunderbuss for a few moves (two or three at most I'd say) wouldn't look that out of place imo. Now the helicopter pack which is generally suggested as his recovery admittedly clashes with the King persona more than the blunderbuss does, but if used for the recovery and nothing more it wouldn't be that much of a deal breaker imo.
When it comes to fan-made K. Rool movesets I feel that there are often ideas to use even the most obscure source of moveset potential using electric pods, boxing gloves etc. to create MUH UNIQUE moves for the sake of referencing source material and I'm don't think that I agree with this moveset design philosophy, because at some point you're going to break his thematic coherence as a fighter. I personally have an accurate idea of how I would design him if I had to make a Smash game and the only visually unique things outside of his King persona I would give him (so no counting generic moves like the ground pound from DK64) would be him using the blunderbuss for his neutral special (status gases), side smash (spiked kannonball), and maybe his grab (the vacuum thing), plus the helicopter pack up special. That's about the same amount of "out of place" moves as Kirby actually has in Smash... and while we're talking about him, at first I was disappointed when I played Triple Deluxe to see the sheer amount of potential he could have from those copy abilities instead of the mostly generic looking normal moves he has in Smash but I have to admit that giving him too much visually unique moves as normals would break his thematic coherence as a Smash fighter. Some of his normal moves are actually inspired from his several copy abilities (like fighter jab and forward smash, yo-yo dash attack and ninja up throw) but they look generic enough to not break his thematic coherence. To give another example there's a reason why things like Gust Bellows, Beetle and Bombchus are regular items and not part of Link's moveset, because his main weapon is still his sword anyway and he only needs his most iconic secondary weapons to be done justice.
I agree with your point overall, as seeing King K. Rool using a mix of blunderbuss, boxing gloves, electricity, etc. moves for all of his moveset as a way to design it with the same state of mind as Mega Man by trying to make each move reference something very specific is not going to feel right; it works for Mega Man because he's a robot specialized in permanently stealing robot master weapons, but I doubt it would work well for K. Rool. I still think it could work wonderfully with the same state of mind as for Kirby's moveset instead, but I wouldn't be surprised if that's going to be the main reason for his exclusion. Especially if we're getting a Kaptain K. Rool Mii gunner costume in a future update...
