I thought it was even more visible how snubbed AFO is on the covers if you look at only the villain covers (Front covers). The red circles show the solo covers, the gray circles show the "versus" covers, which give focus to these villains.
The picture is even funnier when you add back covers and realize that great impact villains like Curious or Slidin' Go got illustrations.
I'm truly wondering if Horikoshi is embarrassed of one of his main villains or is he trolling the "Demon Lord"-wannabe to send him to the margins of the story.
For a full picture - there are 2 color illustrations with AFO:
Ch 306 color page
And a WSJ cover: