Luffy messes around against weaker opponents often, that's not a surprise. Saying Zoro could've beat Caesar is like saying Luffy could've beat him if he was more serious the first time (in fact it's not like anyone aside from Luffy or the weaker Straw Hats fought Caesar so not sure why he's a good example). The arc has to keep going so Luffy, who's perfectly capable of finishing things doesn't take things seriously initially and Zoro reminds him to while being occupied with other stuff. Not strange, Luffy's fine and it's a bit pretentious and ridiculous to insinuate the most hilarious thing to Oda is having Luffy defeat the big bad pretty much every arc even after losing and sometimes letting others give it a swing.
