I would say all of the Straw Hats have fallen off and become just one-dimensional flanderizations of their former characterizations, and some like Zoro and Robin have just become painfully dull since one's always brooding and the other is just standing there when they're not fighting, giving exposition or eating. However, Sanji and Luffy have probably fallen the most out of all the characters in terms of having become the most one-dimensional and so distant from what they used to be and they only seem to exist for overused gags rather than as actual characters.
To me, Oda's habit of spending more time characterizing arbitrary side-characters and their backstories more than the main Straw Hats themselves may be a sign that Oda just isn't attached to the Straw Hats as he used to be and may actually resent them a little since dedicating himself to One Piece to keep the JUMP money train going has deprived him of the chance to focus on the Giant Robot manga and other stories he wanted to write but now can't because he devoted too much time to his bread and butter series.
Oda hasn't even done a single Straw Hat Theatre since 2012, and what very few omakes have come since then have only been done on request from fans or the magazine for holiday events.