lol...you must go in Feb..
I'm just back, and nowhere near 30% of women were wearing crop/sports bra/bikini tops. I did not see anyone wearing a bikini top at WDW. I did see a handful of young ladies wearing tops that tied in the front, but they stood out to me, in large part because they looked a little self-conscious.
I have seen bikini tops at Universal. They tend to be most common near the water rides in Islands of Adventure, but then even at Universal, people tend to either add a top or leave the park (after the water rides).
Tank tops were very common at both properties, especially the athletic style tank tops. (Nike? Under Armour?)
I just went back through my photos, and the majority of folks in the background are wearing regular t-shirts, even the teens, a few are wearing sleeveless tank tops. One woman is wearing a halter-top.
I did see some women wearing a crop top and bike shorts/leggings, or a crop top with high rise shorts.
One thing to keep in mind though is that this time of year, afternoon storms are very common. Whatever you opt to wear, it might end up very wet. When it rains this time of year, it doesn't always last long, but it is often very heavy. You get drenched in just a few seconds.
In some cases, you can wait it out in a gift shop. Other times, we have entered a ride under sunny skies/start of clouds. When the ride (Buzz) was over, it was pouring and the CM's kicked everyone out of the ride exit. If you get caught trying to get to an ADR in the World Showcase that can also be a drenching experience.
For example, Weather.gov for WDW (zip 32830):
today 80% "t-storms"
8/31: 70% "Showers Likely"
9/1: 70% "Showers Likely"
9/2: 60% "Showers Likely"
9/3: 50% "chance T-storms"
This time of year, water tolerant shoes are also a great idea. We tend to prefer quality walking sandals, like Ecco or Teva.