I do wonder if that's the (admittedly very minor) price to pay for semi late stage DLC. The in-game model and render will be polished up well enough, but the Smash team is working on a deadline with less people, so if this part or that looks a little weird but not enough to be jarring, they might not have time to do all the touch-ups they might want, or they might not even notice the things that look a little off until the characters are done on account of them working to get them done. Personally, I think Arsene in Joker's moveset looks a little stiff and not as lively, and I do think he might look a little more natural if Joker had been base game and they had time to make him a little more fluid in his movements, give him idles, etc.
I do think Banjo's jaw looks a little wrong in the render somehow in a way I can't explain, and I agree that both of their eyes look a little soulless in a few bad shots of gameplay, but I'm 100% sure that those issues will be resolved if come next Smash game Banjo-Kazooie are on the base roster.