Viktor is the highest playrate toplane, because he fell into a particualr niche. He's decently tanky with an on-hit effect and a unique damage item.
This is a result of kelpto, tank itemization, and his particular design... not AP.
As well, 9/10 of the most picked champions are not AP, and i could exclude viktor from that list for the previously explained reason, but I'll be generous.
Outside of that you get LB at 15, then Grag, Nid, Elise, Kass, and Liss being the first rep as an actual mage style instead of more an assassin one... then lux at 28, zoe at 33... I've made my point. (I skipped akali, because her ban rate makes it pretty obvious that's nothing to do with her primary stat...)
AP isn't broken, most champions people are playing are AD. However, because of this, and the fact that AP tends to result in scaling magic damage. It's advantageous for a team to have someone who scales well with AP, but that doesn't mean that it's advantageous for a champion to be AP.
Every AP champion would see a massive buff, if they were to remove all AP from the game and replace all scaling with AD. However, if they were to do the converse of that and convert all magic damage into physical damage. You'd probably never see a competitively picked AP champion again. Yeah you might see some who are extremely safe or overloaded, but again those weren't really being picked for being AP in the first place. Take away their magic damage, and you've pretty much taken away the only reason to have an AP scaling champion on your team.