She's really good in solo queue because you know one of your teammates will rarely feed, and that's fucking huge in and of itself.
She's good in competitive because Riot decided to add what I call a Gate (because your best damage/utility output is gated behind a specific requirement, like having to wait a second for Viktor's aftershock) or flat out nerfs to a lot of top tier champions' abilities. Since Ahri's gates (landing skillshots and the front/backswing of her Q) are quite lowand she can easily outplay others', it comes as no surprise she does well against them.
That being said, the best thing to do would probably be to reduce her safety, since that is causing the most frustration, and since it's more of a lower end of the ladder issue it'll be less impactful on the competitive level, and her issues aren't prevalent in competitive at all since it requires her being at a 54% winrate in solo queue for her to be as good of a competitive pick as the 47% Viktor.
Possible fixes would be
-Mana cost nerfs
-Sustain reduction (not recommended)
-Increase ulti CD
-Remove Q MS buff