The joke, in a very poorly communicated way, is that Sora is irritating, bombastic, and hard to manage. The idea of Aqua sharing a similar nature is worrying to Riku.
There are lots of jokes at Sora's expense in KH3, and unless they're delivered by a duck named Donald, they don't land. Everyone jokes that he's rash, reckless, and airheaded, but 9/10 times he makes the right call or throws himself in harm's way for the greater good. We only have a few scenes where he's the bonehead everyone seems to think he is all the time.
Sora getting very excited to destroy the computer ("Brilliant!" -summons keyblade-) in Monstropolis, or threatening Mike with his keyblade are moments where these descriptions seem to work. Otherwise? KH3 gives us a more levelheaded Sora than ever. He's got a grip on who he is, doesn't get enraged by his enemies (he seems to like joking at them better), and isn't in a big fat rush to go fight Xehanort. He just seems to love life, which maybe everyone else thinks is childish? He loves snow, he loves sailing and being a captain, urban environments, robots, dancing, gardening... He's a fully fledged person. Maybe Nomura thinks that's silly, but over here in the west, we just see Sora as a more fleshed out character.
