I think the thing with re-used assets is like... recycling stuff from an old sketch, if we'll use artist metaphors here (and boy, as an artist, I love those). You've sketched a character from their original design that you made a long time ago and didn't complete it, and now, you're working on something at a grander scale. Instead of starting with a new sketch, you're like, "Hey, I actually did something that might fit with this," and you can take either portions or the entirety of that sketch to make something new. It uses assets from the original, both the sketch and your final piece, but your sketch is a framework you'd already made that could be recycled. It, too, is very faithful to the original design and original assets, and it can be helped to shortcut things.
I've done this too many times, which is painfully common when you make a million things that you never finish.
Whoopee, failure to be dedicated to WIPs has its benefits! It's not like it's disparaging for artists to do that, too. Even the most skilled artists have done this very thing. They can pick from the original sources and from prior projects to make something fresh a reality. I truly feel like that was done at least with King K. Rool's costume's assets, as they mirror the playable K. Rool quite well. You can also see the inspiration from both of his older designs, the one from Rareware's DKC series and the one from PAON. It was all used to make the beautiful concoction we have today.
tl;dr: Reusing things, even the tiniest things, isn't giving artists less credit they deserve. Rather, it's a commonly integrated part of an artist's life, professional or not. It's just logical. It can even boil down to just reusing and sprucing up texture maps or pieces, which, in the case of Geno's outfit, is definitely possible. They already have a basis with his cape and hat. Why not tweak those and give them a fresh coat of paint? They've already got some groundwork there, and they can alter the already done models to best fit the real deal. It's not like it's an impossibility, and in fact, it could potentially make the workload easier.
Also, unrelated, buuuuuuut...
THAT MARIO MAKER 2 UPDATE IS MAKING MY ZELDA II-LOVING HEART SING.