Actually, the mermaid theme is to promote this year's Doraemon movie, Nobita's Great Battle of the Mermaid King, which will be in theaters in Japan in early March. It's also the 30th anniversary of the first Doraemon movie (Doraemon: Nobita's Dinosaur, from 1980).
Does that mean that the JAL is getting paid for the advertisement? Possibly-- they're also running Doraemon-themed tours to Okinawa, although I'm not sure why; Doraemon's creators were born in Toyama and I can't find any obvious connection. But on the plane they'll have a bunch of Doraemon entertainment, as well as merch in their in-flight catalogue.
Wacky. Here's hoping they're getting paid enough to handle some of their debts. Would you fly on a plane like this?