Short answer: she's getting buffed because the changes a few patches ago were meant to be power-neutral, but didn't work out that way.
When she wins, she wins harder, and when she loses, she loses badly was the goal. No more EQWRIgniting the ADC, only to watch them walk away with a few hundred extra health, and no more outrunning skillshots in a straight line Prometheus-style, all the way to tower. The end result was substantially weaker performance by lower players, and slightly weaker by mains and pros; not exactly power-neutral.
As for why most Ahri mains complain a lot, most of it is continued fallout from 5.2, when Rito forcibly changed her playstyle, and she went from pro-viable but not dominant to near-broken at low level, but troll-pick level in pro play (Trusting Faker's word on that part). Add three years of devs ignoring complaints about the change, with the exception of repeated nerfs targeting everything but her tacked-on safety, and a lot of the old players outright left. The remaining ones are still mad about their champ being deliberately reworked out of high-end play, but argue continuously with generally newer mains who showed up after the changes. By ignoring complaints for three years, Rito left themselves with a no-win scenario.
For the skin part, two problems. First, if you go through the patch history, Ahri is traditionally an exception to the skin rule: she gets compensation **nerfs **when skins come out, meaning she's probably not getting one unless they decide 15 mana and .4 seconds is too powerful the patch after. Second, while she's usually added to whatever skin line comes out in late Auigust/early September every year, Rito has a bit of a vendetta against Pool Party Ahri. Combine this with the fact that her last skin got more complaints than the simultaneous Vayne buffs, and they're more likely to give her nothing this year than give her a Pool Party skin, the only known upcoming line. Only to realize that people are so used to complaining about any sentence mentioning both Ahri and Skins that they'll keep going no matter what happens.