Only Lad.

I love your Mob meta so I am wondering: do you think Dimple knew about ???% or was he finding out about it same time Reigen was? (I don’t think Dimple ever saw ???%, only 100)

No. I think almost no one knew about ???%. Mob was trying so hard to keep that side of himself hidden and subsumed because he was so afraid of it, and he mostly succeeded.

The only people who knew about ???% are Ritsu (who convinced himself it was a monster so he didn't have to face the fact that the brother he loved had hurt him so badly), Teru (who convinced himself it was some sort of vengeance deity sent specifically to teach him humility so he didn't have to face the fact that his new idol had meant it when he said they were alike), and Mogami (who iirc was mostly like "Huh. Fucked up if true.")

Dimple didn't know about ???% but he did know something that no other character did: the ways Mob's explosions actually work. Because it's laid out for us as the audience, I think it's easy to forget just how little anyone else within the fiction understands about what's going on. Sure, some of them can tell that the kid is pretty out of touch with his emotions, but they don't know that he's actively repressing them and they don't know why.

But Dimple does! Mob tells him during the LOL fight! "This is what happens when they come out." And so while Dimple can't "see" the meter in the same way the audience can, he understands it. He knows that Mob's explosions are outbursts of suppressed emotion, and so he does his best to find the right emotional appeal to either calm him down (the fire) or push him towards something (100% Courage in Mogamiland).

So no, Dimple didn't know about ???%, but I do think that he understood as soon as he saw him. He knew about the explosions, he knew about the repression, and he knew first-hand just how hard it is not to get carried away when you're dealing with that much power, so he was able to be the friend that every part of Shigeo needed.