Law not doing anything actually makes perfect sense
And it would be actively out of character for Kid to give a fuck.
The problem was the Strawhats. Sanji was the one who decided to go after him, okay, that makes sense. He's much stronger than Robin and better in the air than her, and to be frank it's better that Sanji fails in base than Luffy fails in G4. And the result was always going to be failure. I don't think there should be any complaints about Sanji being the one Oda chooses to mount the rescue. Plus the intervention of anyone outside Luffy, Sanji or Robin leads to Momo plummeting to his death- what good is sniping the guy holding Momo out of the sky going to do?
But the cloud didn't seem all that threatening. And Sanji seemed pretty close to Kanjuro. Making his apparent decision to run back to cover Nami a very stupid one, not helped by the fact the panel of him covering Nami seems to be a gag one.
What Oda should have done imo was show that the clouds produced gales of wind that blew Sanji back, or covered a much bigger area. Give an actual reason for Sanji not being able to get after Momo other than apparently choosing to give up while he was almost there and run back to cover Nami.
Not a case of catastrophically bad writing, but very clumsy.