I think the point Horikoshi is making is reasonably potent, that political forces frequently stoke and ride the anger of the masses for their own gain. Making Spinner into a big stupid brute who can be easily controlled is a great way of silencing the actual opinions of the symbol/figurehead. It's definitely tragic, but it makes a ton of sense from AfO's perspective.
Earlier, we saw Spinner uncomfortable about being pushed into the public eye as a symbol for heteromorphs. As much as he was ostracized, we mainly saw Spinner looking for companionship from Shigaraki and the others. AfO simply capitalized on Spinner's desire to be like Stain (a symbol) and his own feelings of powerlessness from the MVA and Raid arcs. I really don't think Spinner would be able to offer much in a "battle of wills" because he was always the one who had the most doubts about AfO and Shigaraki's greater plans. We've had hints of this anger at racism against heteromorphs in Spinner, but it was a secondary characteristic that played more into his desire for belonging and acceptance.
So I think this works well as a plot development because AfO gets to use Spinner for his own gain while also removing a source of friendship for Shigaraki. Spinner, as maybe the least villainous League of Villains member, becomes an object of our pity and a much more concrete symbol of AfO's evil than any of his large-scale plans. I find it to be an effective form of telling this tragedy, even if the sudden focus on Shoji is a bit abrupt.