Which is lower than the reactions Luffy's red roc got.
Since you love tiny details, we have
- Kaido and Big Mom shocked by red roc's damage
- Zoro himself questioning the level of Luffy's haki
- Kid surprised by Luffy's haki level and that a punching attack can hurt Kaido
- Killer literally starts believing they could win because of that
Hell just from this, Kaido started to wonder if Luffy could actually match him
No you're reading it backwards.
- Big Mom went out of her way to
save Kaido after Killer's attack and literally stated they
underestimated all of them after Zoro's attack failed to land
- The reason she didn't comment on Luffy's attacks after the red roc is because
she already knew that Luffy's attacks could damage him. The moment red roc damaged Kaido, the surprising moment of Luffy's lvl3 hardening damaging Kaido passed.
- In Zoro's case however,
she didn't think Zoro could actually cut him, hence her surprise at Zoro's attack actually cutting Kaido