Ahri honestly isn't a braindead champ. Now, before you call me an ahri fangirl let me make a few things clear.
I learned to play league with Ahri. This was back when everyone was playing her dfg assassin. However I played her as a kiting mage, aka what she got turned into.
I am currently 8 mastery points away from having mastery 5 on her. She is also my default blind pick mid.
Now, I do not main Ahri and I don't think I ever have. I began playing league on Ahri but the first champ I actually tried to master was Ezreal. I currently main Taric, Naut, Thresh, and a little bit of Nasus.
I have mastery 5 on all of those champs and have had mastery 5 on them for months. The only reason I have such high mastery on Ahri is because of the 3 midlaners I am comfortable on, she tends to fit team comps and be a safer blind pick than the other 2, Veigar and Xerath.
Ok, now that I have given my qualifications to be able to knowledgeably talk about Ahri, and proved that I am not a massive fangirl, although I do like the champ, lets discuss why she isn't a braindead champ.
Ahri has a low-mid skill floor, aka how hard she is to play decently. This is due to a number of reasons. Her passive gives her good sustain in lane, her waveclear is good after a few levels, her w+r dmg is targeted and pretty high, although capped. Early on she has an advantage in aa fights to the death as her w doesn't interrupt her aa animation. Pre 6 her orb gives her decent mobility and true damage. Post 6 her ult is on a fairly short cd and gives her 3 dashes.
Now, all this points towards braindead no? Well, let me explain why she isn't braindead.
A big part of Ahri's damage comes from her Q. This ability serves as poke, waveclear, sustain, escape, and about 1/3 of her all in damage. Having all this packed into a single, average mana cost spell seems like a good deal right? The answer is yes and no. On one hand it is hands down the best ability to max. On the other hand you have to constantly think about how to use it. To get passive stacks? To use passive? To waveclear? To poke? If you push then will it be up in time to use it to escape a gank? Do you have enough mana to do all of these? Or do you have to conserve it.
That is a lot of pressure on just one spell. But lets move on to the next. W. Foxfire is good for burst, but it does have a damage limit. In a 1v1 fight with no minions it is better to hold q for a few seconds if your passive is up because w heals more vs a single person. So deciding whether to use q or w when your passive is up is a split second micro management decision.
Her entire passive is micro management actually. It isn't easy to keep track of in a fight but it separates a good Ahri from a bad one.
Charm. Ahri's signature move. One of the most powerful cc's in the game even though it is only single target. This is offset by the very obvious spell animation, slow travel speed, and small hitbox. Sure, it is powerful, but as far as abilities go it is one of the easiest to dodge in the game.
Now her R. The part of her that people love or hate. It is also what separates good and bad Ahri's more than any other part of her kit. The single target dps is incredibly low as far as ults go. If it is hitting 3 people each time it does a good amount of damage at the cost of putting yourself in a dangerous position. A low skill Ahri can look flashy darting around a fight quickly, but they are playing like trash and will probably die in a few seconds.
So. Ahri does have a fairly low skill floor. However when playing her like most people do she is honestly a pretty weak champ. However she also has a very high skill cap and the ability to snowball decently while remaining semi relevant even if behind.
Anyone can play a champ decently while not thinking. Said champ having a low skill floor doesn't mean they are braindead. It is when a champ has a low skill cap that they are braindead. And the difference between a good and bad Ahri is similar to the difference between a good and bad lee sin.
Sorry for the long ass post, had nothing better to do with my time and got carried away.