I think there's something underappreciated here in the fact that the premise of the entire manga not only justifies the typical boob-talk onsen fanservice moment, but actually makes it an entirely expected and logical course to take. This is a manga about girls and their boobs. When do girls most often talk about their boobs? When the girls' boobs are all out and bare. Therefore, the onsen boob talk is actually necessary from a standpoint of desiring good writing. It would do the reader a disservice if it were absent.