She is often depicted along Luffy and Zoro during pivotal events in the story
After the time skip sanji became clearly more plot relevant than her. It's not that Oda made the time skip start with Monster trio together and Luffy claiming his dream in front of Ray, zoro and Sanji for nothing.
I agree that Nami at the beginning was more important narrative wise than Sanji, but right now Oda usually represents the M3 as the "symbol" of one piece, even kinnemin thinks about the M3 when he thought about SHs crew in general.
We literally had Luffy claiming to Sanji he won't become pirate king without him and shut upping Nami, we got Sanji being the one in role at Zou. Nami is the most important female character, and she started as the third most important one in the series, but it's clear Oda made up his mind after the time skip by giving Sanji more screen time, plot, relevance and statements from characters to how much he is vital for Luffy and their relationship.