I'd agree on paper GNR is a head scratcher.
Buuuuut... mainstream theme relevance is irrelevant in the crazy world of pinball. ACDC, Metallic, Aerosmith were all about 25 years out of the mainstream when they got pins.
Also Slash is a pinhead so that reduces costs (see both Spooky music pins) and you get to have Slash help with your game design! Oh boy Slash is visiting the office!