Motörhead is both more infamous and more famous than Slayer... but half the people who wear their t-shirts can't name 10 songs they do.
I own quite a few "Metal shirts" of bands I listen to all the time and I STILL wouldn't be able to name 10 songs. That's just due to the number of bands I like, especially when they are "younger" bands.
It's an entirely different story for the bands I grew up with, some decades ago.
"Knowing the name of their songs" is not an indicator whether someone likes the music or not. But you're certainly right that everything with the Motörhead writing on it became some kind of "fashion item". Fine, hope the band makes some money out of it...