One Piece's excessive goofiness is fine but pre-TS, it knew when to be serious. Hell, even post-TS it knew when to be serious, see Dressrosa.
Now compare Luffy vs Doflamingo to Luffy vs Kaido.
Kaido was a tyrant of the highest degree, so tell me why Luffy's smiling his ass off constantly talking about how much he's enjoying himself and how much fun he's having. Now compare to how much fun he was having vs Doflamingo, Cracker or even Katakuri for that matter.
Idk why Oda decided to constantly relieve all tension in Luffy vs Kaido by having them wank each other off at every chance they got. Then making One Piece into a literal cartoon which pissed away any tension that could possibly have been left.
One Piece's problem isn't goofiness, it's that Oda doesn't know or has forgotten how to create and manage tension. These days, Oda's idea of tension is a cliffhanger that is immediately resolved in the next chapter. Not to mention his reluctance to perma-kill characters.