There was a british localization expert doing a panel talk, and in part of it he detailed how the magazine he worked on at the time had maybe 10.000 subs, and that matched the number of VHS sales for a new anime around the same period for the region. At the same time, the numbers for hentai stuff was around 5 times that, so there was this association of anime being erotic by non-fans who just consumed the erotic content (even if you say those 10k anime fans all consumed the hentai, that still leaves those other 40k folks), but actual normal fans who watched all kinds of content were way fewer and would be drowned out by this perception that was generated by people who would themselves deny being anime fans, but because they were watching hentai in the closet it colored their perception of anime as a whole.
Nami is very much in the same boat.
Nami has B-cups during the Buggy and Airlong arcs. She eventually grew out more, and then after the 2 year timeskip and later became a hourglass bombshell, but if anything the actress they got portraying her is TOO curvy, not not curvy enough.
So yeah, basically the complainers are normies who have no clue about how Nami looked like in the early episodes, likely have only had any interaction with her from fanwork porn, and since the porn is likely both based on her later design and also exaggerating it for effect by itself, they are left unfulfilled by the actress. Personally, Nami and Zoro look totally fine from that live action thing, Ussop looks weirdly effeminate and his nose is a foot shorter, and for some reason Luffy is a mexican now, but Nami is fine.
There's this theory that during times of economic downturns in Japan, content where women have huge boobs sees and increase in production. I dunno how true this is, but it was the closest thing to a justification for the pre-timeskip growth Nami experienced I ever read.
So yeah they're not normies in the sense of being someone with no knowledge of the internet whose hobbies are fishing and playing football, but in the context of knowing one piece.
Finally, you have to compare apples to apples. One Piece has a thousand episodes where none of the main cast hooked up, had children or families and most if not all romance is played for jokes, with the little kid enjoying tight hugs from Robin or Sanji hitting on everyone as his way of greeting them. In half as many episodes, Goku had had 2 children and his child had had a child. In the first episode Bulma flashes her skirt to manipulate him into giving her his dragonball. Naruto on the other hand had the sexy no jutsu where he would transform into a naked girl covered by ninja steam. He also is a daddy now and his son is the protagonist of the current sequel series. Bleach I haven't followed but someone else can let us know. Point is, when contrasted with other mega-hit shonen anime that typically have some degree of sexual gags and erotic jokes as a matter of course, one piece is on the lower end of the spectrum when you look at what % of the show is involving sexual content. You may be using the one drop rule, where one scene just instantly taints the entire show as sexual even if it's 2 minutes out of 2 months of runtime, but no, that's wrong.