Okay real question time: Do yall think Mob already knew Reigen was a fraud before Reigen confessed to it in the finale?
Entirely reasonable argument for "yes" given that ???% calls it out explicitly to Mob. Though ???% is viewing everyone in the most negative light possible and so "Reigen is a fraud and is using you (us)" might count more as a "my friends are all fake and hate me" kind of inside-fear, moreso than an outright full and complete awareness of Reigen truly being a fraud.
(Similar ambiguity around Separation Arc, when all the media is pegging Reigen as a fraud. Although the media scandal didn't give Mob any information he did not already know in terms of the unverifiability of Reigen's powers. And when Reigen prods at this with the "Do you know what I really am?" Mob's deflection of "Yes, a good person," could be read a lot of ways, as far as the true answer to the question he was dodging. Like "I know you're a fraud but more importantly you're a good person, so that's who you are" or it could be "Psychic powers or not doesn't matter, being a good person matters, like you taught me.")
I'm personally not totally sure where I fall on it. I feel almost equally torn right now between "Mob figured it out along the way, and Mob was just continuing to go along with it until Reigen finally owned up to it" and "Mob has his doubts throughout the series, but Reigen could still be telling the truth and there's no strong enough proof for Mob to resolutely decide Reigen is lying until Reigen outright cops to it."