I think you need to separate Manson's notoriety out from his popularity. He was definitely infamous, due to his public image, his fans, and also due to people trying to blame Columbine on him. So people (over a certain age at least) definitely know who he is. But he was never all that popular outside of a certain demographic of angsty teenage boys. His band's most popular album sold 1.9M copies in the US. And while people bristle at the Creed comparison, even if they had less cultural cachet they were far more popular than Manson was in terms of record sales and playing live, having two albums with 6M+ sold in the US and another that sold over 11M in the US.
I think people forget how awful mainstream rock was in the late '90s and early '00s when you had junk like Creed, Matchbox 20, et al selling 10M+ records.
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