After one of the most heartbreaking flashbacks in the story, Sanji tries to become a mouth piece for Nami by flat out ignoring the overwhelming tragedy of racism and the ostricization of an entire people by being a loudmouth who only responds to the fact that Nami's childhood was rough. He showed no personal opinion on the discrimination, no modicum of basic human understanding, he only wanted to please Nami, and his action isn't even what Nami wanted. Immediately after she told him to shut up, he went right by to having no reaction to the situation.
Sanji has little character to begin with, but what defines him in this story is his love for women, which is extremely vapid, and because of that, his character changes at the drop of a hat to fit whatever the nearest woman wants for him.
tl;dr he has no character of his own; the little bit that he did have (his dream of finding the allblue) is completely annexed in favor of being a tool for women. Truly, entirety of FI ruined what little character he ever had to begin with.