Sir Mix-A-Lot did more for womens' self-image than any campaign
I'm going to go out on a limb and say that people care about what others actually think of them, rather than just what they say.
First, yes, body-positivity is a good thing. It sucks to hate the meatsuit you're stuck in. It's good to know our friends and family love us for ourselves, and our bodies are a tertiary consideration at best. But most people want to actually be admired or attractive, rather than to be humored. It can feel nice to hear your friends say your story or novel or fantastic, or that you have a beautiful singing voice. Than you go on American Idol and discover that they were lying to you to spare your feelings the whole time, and you are disappointed.
Body-positivity campaigns like Dove's Self-Esteem Project, and Victoria's Secret Plus-Sized model, have their heart in the right place. They're not bad things to have. But ultimately they feel patronizing, and it's no secret they're in it for the market share. They won't change how you think other people view you.
Sir Mix-A-Lot, on the other hand, REALLY likes big butts. So much so that he cannot lie about it. His like of them is visceral and honest. And, very importantly, his public proclamation was extremely well received. Everyone knows the song, and enjoys it. No one reacted with "Wow, this weirdo is singing about his freaky fetish, let's laugh at how cringe this is." They reacted with "YES!! OMG BIG BUTTS ARE THE BEST!"
At a time when skinny blondes were considered the top-tier body, Sir Mix-A-Lot exposed the preference falsification that had been going on for ages. At last it became acceptable for large sections of the population to admit their true preference for fuller figures and dangerous curves.
This, in turn, resulted in untold millions of women realizing that their body-type was actually very attractive to a lot of potential mates. This wasn't some pretty words said to spare someone's feelings or to sell soap and underwear. It was a real desire that was made manifest in people's actions. You don't need to worry that someone is just being polite when they're in front of you with desire in their eyes.
And thus, with a single extraordinary song, Sir Mix-A-Lot's honest admission did more for women's self-esteem than any amount of multi-million-dollar body-positivity campaigns.